Have you considered a quattro tt? 220hp from the 1800. Yes its not a smooth 6 pot but hard to beat power wise per displacement.

Regarding a bmw. The closest your going to get to what your looming for is a 2.5-3ltr z4 or an e46 330ci but as you said yourself you'll need to change the gearing etc.

S2000 wise as we talked about before they are rising in value now. Even at 6k ypu'll find it quite hard to get a good example. Even at that it will most likely be totally standard spec, which imo they arent great. They need atleast a decent set of wheels, lowered and maybe a decent cat back. Where as the z4 has the 6 spot sound and come woth some nice wheels from factory. S2k's are also not a good daily driver unless your into no luxury's, rattles/road noise, very little low down power which is what you need more of the time. They are very twitch unless set up properly and arent very comfy in general. Z4 has alot more cabin room also. Now i loved my old s2k. Great cars but they suit 1 purpose only tbh

E46...... I was thinking the same as you before i got the z4. Will i get a nice 330ci etc. In the end I decided I couldnt buy an e46 unless it was an m car. They arent like the e36 that way. Thats just personal opinion.

Id love a nice e30 318is or 325 but to find a decent example these days is near impossible and if you do that doesnt need money spent on it they are quite dear.

What about a boxster s?