Plenty of good advice already OP.
Budget-wise any Coupe around £7K will have done 100K miles at least - but the engines seem to be pretty bullet-proof to be fair. Although some people can't deal with the idea of that sort of mileage. :?
I've had 2 E86s (i.e. Coupes) in the last 4 and a half years, although neither has been a daily.
Both were Sport models with the M-Sport seats, and I've done 200+ miles and up to 5 hours in them in 1 sitting without any aches - and I'm nearly 3 times your age! :lol: Nearly forgot, both were on Bridgestone run-flats too!
All 3 litre Z4s have the sound generator - good advice to play about removing 1 or both foam inserts until you get a noise level you can live with on a daily basis. I haven't even looked at that on my current one as it's had a custom cat-back exhaust. :lol:
Springs do tend to break, but new ones aren't dear.
Electric water pumps do fail, and when they do it is dramatic - mechanical ones tend to give some warning by dripping coolant and/or squealing. Electric ones just stop working, so the temperature gauge gets from normal to overheated in a mile or so! And you don't want to drive it far once it's gone or it will go into limp-mode.
The main issue with the pumps is that they aren't cheap - an OEM Pierburg pump from Euro Car Parts is usually around £250 for the part!
BTW I wouldn't see going from a 200 bhp hatch to a 265 bhp 2 seater as a downgrade to be fair!
As I said mine isn't a daily, so most miles are done on dual-carriageways/motorways out of rush hours, but my OBC is showing an average of 33.6 mpg - even though the average speed is 42 mph!
If you want some ride comfort the general advice is to ditch the run-flat tyres - if I used mine the way you plan to use yours I'd look for an SE with the M-Sport seats. The potential upside is that an SE should be cheaper than a Sport, so if you find one with the right seats you might get a good deal and Sport models also came with M-Sport suspension which is stiffer and 10mm lower. Don't buy an SE with "Sport" seats intending to upgrade to M-Sport seats - used ones will still cost you the best part of £1,000!
Sport models had the MV2 wheels (Style 135M) as standard (which are a PITA to clean) or the split-rim Style 108 wheels that corrode around the bolt-heads due to a chemical reaction between the different metals of the bolts and the wheels which lifts the lacquer (I sold my 108s while they were still in good condition).
Some SE models had 107 Ellipsoid wheels (still staggered 18" rims) that IMO look better than MV2s or 108s, but I wouldn't worry too much about wheels - you ought to be able to swap them for a couple of hundred by the time you've sold the old ones!
Bear in mind BMW were a bit stingy with standard equipment on them - Bi-Xenons, heated seats, cruise control, rear parking sensors, electric folding mirrors, and cup-holders were all options so if any of those are "essential" make sure you look for them! Of those, cruise is probably the easiest to retro-fit.
Anyway enough waffle from an E86 fan - might as well end with photos:-