i'd get an early s1 elise or exige, or vx220, pick them up for peanuts these days for what they are.
early S1 had the k series engine but it was a lot more bullet proof in the lighter cars than when it was in land rovers. they did go, but it was down to cheap gaskets. rover saved a few quid per car by using cheap gaskets which blew out over time. i think hangar 111 sell new upgraded gaskets and once fitted i've been told no one who has them ever has a problem with them blowing.
if you get one and they don't have the uprated gaskets i'd do it when you buy it as it would be cheaper. if you wait for it to blow you'll have to have the head re skimmed and your looking at about £500 ish for the job.
Vx's don't have that problem, don't know a huge amount about them but speak to ekona as he had the turbo version or some of the other lads and they'lll know of any issues with those.
if you want to do an engine converison i know a very good man who'll drop what ever you like in the back of an elise, he race tunes and preps elises for the elise cup.
or as mentioned things like the caterham are great fun