Hard to fit one. lmao It's near on impossible.
You can fit one, but you WILL be a lot better off buying a car with one as standard.
Visit www.vectra-sport.com and see if anyone on there has done it yet, as I considered it about 18 months ago, but gave up due to the amount of hassle it is. I did a reply to a similar thread on Vec-sport too (where my username was David P (David)).
There are some that come with the fuel computer, but then there are some early sris that don't have the 20mm lowering springs.
(Won't be a problem if you're going to lower it anyway though.)
Go for a v6 sri or cdx if you can afford the insurance (and 25mpg ish worst case scenario). 140mph in either is possible.
Get a black top bumper one. It'll have the switch for turning traction control off (dash light on=traction control off). You'll want that for driving it properly. (Nothing worse than going round a bend, having a wheel start to slip on the apex and having the tc cut in, removing the power and the fun. Dull as...)