The reason the early ones leak is that the seal over the windscreen was a poor design, relying on the pressure of the quarterlight against the rubber to seal. Later cars have a ridge down the A-pillar seal that folds slightly around the front edge of the quarterlight to make a more positive seal. It's easy enough to tell visually if a car has the later seal. They only leak at speed as I understand it anyway.
Anything relevant to S2 (or Turbo) is relevant to the cab in mechanical terms. Cab specific issues are all to do with the hood as you'd expect. Seals are expensive, but they can be visually checked and they are usually still in good enough condition. Hoods themselves wear around the top of the B-pillar and the first signs are that there is no stiching in that area as it wears away first, then the fabric after. The mechanism is always electric on UK cars and is quite noisy and not very fast, but it should work reliably (you might need to give it a nudge after unlatching to tell it it isn't still locked closed when putting it down). They aren't very noisy inside when driving for what they are when it's up, and when it's down if you have a windbreak they are pretty pleasant inside at any speed you'd be comfortable driving, and possibly quite a bit faster, too. The back window might be cloudy - you can resurrect mild cloudiness with something like Renovo, but if it's bad it needs a new window. The window and even the rear section of roof can be replaced separately, but I can't see how you'd ever get a match between old and new fabric. A complete hood is about £1,000 fitted. If the car you see has anything but black or cobalt blue hood it's an aftermarket replacement (not necessarily a bad thing) or an import from a market with more flamboyant colour options.
Aside the A-pillar seal mentioned above they shouldn't leak except maybe when spraying a hose around the window seals when washing. The will leak when parked nose-down on a slope in heavy rain because water collects in the boot lid aperture and weeps through a plugged hole into the car, but it's a cheap fix to replace the 2 rubber bungs in the boot drain channel with some that have drains in (from a 964 as I recall) and some fuel hose to lead the water to outside.
In terms of practicality don't even think about LHD for the UK as it will be somewhere between highly inconvenient and outright dangerous with the roof up as there will be a massive blind spot right where you need to see to pull onto a roundabout for example. The boot is the same size as the coupe, but very shallow (maybe 8") - you can't stand up bottles of drink/milk etc. in there if you go shopping in it, but you can get 2 golf bags in for example. The rear seats are quite a bit smaller than the coupe also as the side panels are much thicker to accommodate the mechanism. The chassis is noticeably more flexible than a coupe, but still not bad in absolute terms. It's best to avoid stiff suspension and bigger than 17" wheels for that reason. They are slightly heavier than a coupe, but the extra weight is all low down and they have a lower roofline, so performance is much the same. The lower roofline means the screen is shorter so they feel much lower inside than a coupe. I believe they have a second floor for strength, but I couldn't detect a lot of difference inside the car in that aspect so I guess the extra bits are underneath the original.
Not many have air-con, but to be honest you don't need it as the cold air intake and fan are enough when the roof is down (and you aren't going to drive with the roof up in the sun, are you?) Leather is a nice thing to have, and most do, even if it's just because it's a bit showerproof and bird plop cleans off it more easily than with cloth. It can be hotter when you get in though, of course. Very few seem to have sports seats, but they are sufficiently better that they are worthwhile if you were to find two similar cars in other respects.
I used mine for one summer and two winters as a daily driver (50,000 miles in that period) and it's fine for that, so long as you don't have to carry more than two people very often (or the extras are pre-teens, possibly) and you can cope with the low, flat boot.
