as far as i know mate yea you keep the original engine although youd have some expensive modifying to do.
Theres a place near me in derby called top boss performance. They are superb. they can do anything to your car, they stock every performance part possible. When i get a valver this year im going down there for a full Brembo brake conversion. Theyre not the cheapest around but you are paying for expertise and experience. (theyve always got 4-5 cossies down there and i saw a lotus V8 the other day) They had a project car which the owner runs around in, a 555bhp sierra 4x4 cosworth! Anyhow, they said that it would be cheaper to supercharge a valver, than to turbocharge it! The price for a supercharger they said would be £3000. I havent asked anywhere else so really i dont know how much of a competitive price this is, but i imagine you arent going to get it a vast amount cheaper.
As for BHP gains, you must be looking at kicking out (in total) 190-210bhp? Dont quote me on that but i would be very surprised if it was less. I guess it would give you a nice wide spread of torque and with a williams i would imagine that 3rd and 4th gears with a turbo fitted would be superbly quick.
The best thing to do would be to go on a rolling road and find out what its kicking out now. One more option, that would be cheaper than these two would be nitrous! £4500 is alot of money mate, it could get you a hell of a lot of performance goodies, brakes, cams etc on what is bog standard and a very quick little car, which could give almost anything a good run for its money.
Best thing to do with a williams (apart from ss exhaust and lowering) mate is leave the engine bay and exterior styling alone. They are more valuble when left bog standard and its getting harder to find one that hasnt been messed with. I saw one the other day with a bad boy bonnet, why on earth youd want to do that to an already aggressive looking car is beyond me. If one came up in my rear view mirror, bog standard, id move over!
Cheers mate
Pete