[msej449]

I live down on the South Coast and have a car about the age you're looking for (not for sale!) so I'd say there's nothing that's a Big Deal to look out for, unless it's above-average mileage. In that case, as mentioned, above the 40K miles mark there are a few things you might consider replacing (charge pipe etc.) but otherwise nothing fundamentally flawed.
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All other things being equal, the HK Audio is still worth having above the base if it's on offer. And personally, I think that the Exterior mirrors - folding with anti-dazzle were worth the £275 extra.

I went for the full Professional navigation because I'd had the base one on my previous BMW and I liked the way you could split-screen the bigger display. After 3 years, the Connected Drive service is an annual subscription, for which you get 1. UK map downloaded to the car automatically, 2. Euro map updates downloaded to a USB from the portal, and 3. RTTI real-time traffic info. So for 3+ years, it would be good if the owner has a subscription running and you'll get all these services until it runs out and you can decide if you want to carry on. It's about £60/yr so up to you to decide if it's cost-effective.

The full High-beam Assist plus Adaptive Headlights plus Xenon Headlights are very good and if you commute cross-country in a UK winter these are definitely worth having. These were three separate options. Some dealers didn't realise so people ordered what they thought were the full adaptive xenon thing but came out of the factory with just 2 of the 3. You need to see the original spec' sheet to confirm what's actually fitted: it's not obvious externally. All of these options were in the Visibility package plus headlight wash (a waste of time IMO) if you went for this £440 pack.

Through-loading was an option when I bought and this make a big difference to the usability in terms of being able to carry longer items. Because it was an option, it often got missed-out on new builds because the dealer didn't bother to mention it.

People argue forever on whether the Adaptive M Sport Suspension is desirable or not. I like it, but the difference between the modes might not be as dramatic as you expect: perhaps worth arranging a non vs fitted test drive, if you can, to see what you think. The presence of the 'eco pro'-'comfort'-'sport' rocker switch by the shift stick is no indicator of whether the care had this: on a car without the adaptive suspension the switch will just change the throttle mapping, not both the throttle and the suspension.

Comfort Access Ignore the UK press hysteria about this. Most UK premium/performance car thefts are 'to order' by professional gangs, so whether it's conventional key or comfort access is irrelevant to them. The M235i is down with the average car in terms of probability of theft, and the primary risk is if it's parked on your drive (2x more likely to be stolen than on the road and 3x more likely than if garaged). Anyway, I find it useful and you can't lock your keys in the boot by mistake.

M Performance Limited Slip Differential LSD: This was only a retro-fit option from the dealer, costing around £2500. If you come across one where the owner has retro-fitted it, then this is a really big deal and very desirable option on the M235i. If you're going for the M235i for it's performance and handling then this makes a very big difference to your enjoyment of the car. Probably more than any other option. Odds are very low you'll find one. If you are lucky and get a car with LSD on, you will need to check with your insurer ti see if it's classed as a modification. Some insurer's do and some don't. Mine does but we're only talking about £28/yr extra.

'B63.21.2.337.710 079900 LAMP SET' Ask if the owner is including the standard spare bulb set for this model. It's a pricey item at £80.80 +VAT. Apart from being a useful spare to carry 'round, it's mandatory to have it if you take the car abroad.
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If you decide on a convertible there are some items you'll want to look out for. I know you said a coupé, but I'd say take a look at the convertible - I found that I put the top down a lot more than I expected especially in spring and autumn, and it transforms many otherwise mundane journeys.

Reversing assist camera plus Park distance control, front & rear are pretty essential because the rear window is smaller than in the coupé. You regularly see threads on here from people who are trying to retro-fit these and really, for the hassle, you'd do much better just getting it as original equipment if you can.

Wind deflector was a £260 option that really transforms the cabin on a longer drive on faster roads. Makes the cabin much quieter and also good for security: stops items on the back seat from blowing-out and when locked, hides anything there.

'Convertible Roof Headlining Campaign' There was a late 2016/early 2017 non-urgent recall to replace a minor component of the convertible soft-top mechanism (the spring, I think). Didn't cost anything, so just worth checking it was done. It should show up on a customer Service Invoice with the above description.

General Soft-top Roof Care Ideally, you'd keep a soft-top in the garage. In any event, worth trying to determine how diligent they've been in cleaning and proofing the roof. I re-proof the fabric twice a year, and perhaps if it's really badly hit by tree sap or bird strike (usually once or twice a year in addition). But overall, the BMW soft-top is very good, and a massive advance in terms of durability and soundproofing over previous vehicles I've had.
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The rest are options not particularly specific to the model and which you'll have to do the usual 'is it worth it?' calculation if they're on offer. If you go for the High Gloss Black interior trim (which I actually prefer over the retro-fit carbon fibre) just bear in mind that it's easily scratched by hard bristles e.g. the usual vacuum cleaner upholstery brush. I have the Oyster Leather upholstery, but like all lighter leathers, it can take on the blue from jeans - however, I've had no trouble cleaning this off (there's a BMW leather cleaning 'kit' you can get [B83.12.2.288.917 £23 incl VAT] or just do a search on this Forum).

Mixing BMW & non-BMW Screenwash - Dont! Lots of threads on this across all the forums. The owner's manual is explicit in warning you not to mix BMW and non-BMW screenwash. There are plenty of reports of people doing this and 'gunking' up their screenwash system. The 5 litre BMW Screenwash [B83.12.2.409.032 '086506/S/WASH 5L' £18 incl VAT] is more concentrated than the usual Halfords/garage stuff, so the BMW works out at about 2x the cost once diluted to the same strength. Given the hassle and cost of 'de-gunking' it doesn't seem worth the risk for the £8-£12/yr you save using non-BMW. But it looks like you can get away with 3rd-pary if you fully-flush the system - but but if you have the car serviced by BMW they'll top-up with BMW concentrate, so beware. So always ask what the seller has been using ...

Connected Drive App Pretty naff in my view. The app's main value is that you can put a destination in and then transfer it to the in-car Nav and you can lock/unlock th car remotely. You can also locate it when parked (but bizarrely, not when moving). The app integration is poor (TuneIn etc.): it is truly cumbersome and often doesn't work. Things like fuel status only show the tank level when you last (a) wire connected to the USB port and (b) opened-up the Connected Drive app.

Hope this all helps.