Silver 1999 S1 111s is what you want....but then again I'm biased

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B&C 160 upgrade i think was basically head work and cams done on standard car. I believe typically producing around 145bhp.
S1 135 is a nice car but you really need to search out the relatively few later ones which have the 111s close ratio box and 111s seats and wheels (although they are just cosmetics really).
Edit..that's rubbish...they all have c/r boxes)...
The 111s and 135 have very little to choose between them aparently. I haven't driven a 135 so can't say which is best. I really like my 111s though and I imagine that you'll find a 111s easier as there were a good few more built.
If you are set on a S1 looking for S2 bilstein suspension upgrade would be a good idea as they cost £500 or so plus fitting. Most cars still sat on standard red Konis are probably at least 6 years old and a bit tired.
Without wanting to confuse you further...have you thought about a 160s. Not to be confused with B&C upgrade. 160s was last of S1s has 160hp ish non vvc with nice lumpy cams, victory wheels, LSS and corbeu (sp) seats.
People will tell you they aren't much good for day to day use but I can tell you that they feel a bit more special than the others and although I don't regret going for my car, if I was to choose another one now I would have a 160 s.. (there are two different factory 160s too...so you need to research which one you want...later ones had different ecus and the slightly more gucci seats).
good luck...