What they said, but basically, yes, personal preference. Coming from a Z4 you won't be disappointed with either.
You'll find lots of cars have had the brakes upgraded anyway (disks, pads and hoses). That's fine, don't be put off. Very few cars run standard brakes.
Standard suspension is better on the S2 (Bilsteins) than on the S1 (Konis), but lots of S1s have had the S2 kit fitted (which works reaaaaly well!). Incidentally, that red 111s I linked too is on the originals, so budget £600 for that. Konis only last about 20k miles, so those are overdue if they are original. Nitrons or Ohlins are expensive upgrades and are good for real trackaholics. Keep an eye out in the ads for those.
Both S1 and S2 are just as likely to cause trouble! It's not always true that the older or higher mileage the car, the worse it will be. It's a bit of a gamble really and they ALL need love and attention. You can forget your German engineering!
Look for a car with an enthusiastic owner. Better to buy one which has been tracked by an owner who knows how to keep it properly than one which has had "one lady owner" and a full history, but has never been warmed up properly or had it's oil changed between services.
If you're going for an older Rover K-Series engine, make sure it's had a cambelt service recently or factor in £500 for that. Later S2s with the Toyota engine won't cause any bother as long as the book is stamped.
If the car still has a standard exhaust, budget £400 for a sports one. You might think you don't need it, but sooner or later you'll hear another Elise and you'll get "exhaust envy". Yes, you will.
Any cars you like the look of, just post them up here. SE is never short of opinions!
P.S. Don't buy this one.
http://pistonheads.com/sales/1077057.htm 