There are only about 1200 Williams in the UK, so your best bet is to look for an early 2.0 Megane engine, because its the same engine and there are more 2.0 Meganes about. Be careful though, because they stopped using the F7R 150bhp 2.0 in the Megane after a while, and started using the 140bhp 2.0 from the Laguna - its not an F-series engine (16v = F7P, Willy/early Megane = F7R) and doesnt make any more power than the 16vs 1.8.
The reason the 2.0 is really the upgrade to do to the Clio 16v is not that the 2.0 makes a lot more power (only 13bhp @ fywheel), but becuase it has 85% of its torque at 3000rpm - whereas in the Clio 16v you redline it all the time!
Its a bit of a hit and miss game as well - some standard 2.0 F7R engined cars make up to 140bhp at the wheels, whereas some only make a lot less. The 2.0 isnt any easier to tune either: it has the same inlet manifold and head as the regular 1.8 F7P.
For your turbo questions, best to see
http://www.bbperformancetuning.co.ukwww.bbperformancetuning.co.uk. Thats the only forced induction conversion that seems to be going down well - theres no popular or tested supercharger conversion that I know of.