Okay, for that kind of power figure there are 2 options, the 2nd (and more expensive) isn't really worth doing, I'll explain why after.
Since you have a 98 it'll be an AGU engine, with a K03 turbo, so the first option is to fit a K03S turbo from a later model. You can pick these up for around £300, these have a slightly larger inlet flange than your stock turbo, so you'll either need to get your existing TIP (turbo inlet pipe) machined a few mm, or the better choice is to get a silicone TIP (about £120) which will go straight on and flows a LOT better. Now with a 4bar FPR (£30 ish) and a custom remap, you'll be knocking on the door of 230bhp. Anymore than that and you'll need to upgrade the downpipe, CAT and intercooler.. that's when the cost really starts to creep up.. but that magic 250 number is looking more realistic.
The second option I mentioned, would be the K04 turbo from an Audi S3 or 225 TT. To do this though is a bit more work. For a start the downpipe, CAT and intercooler upgrades are pretty much critical. Also, it's not a direct fit, you need the audi exhaust manifold, uprated injectors and some work done with the turbo feed lines and boost hoses, aswell as the custom remap. From that you'll see about 260bhp.. BUT.. for around the same cost as that you can get yourself a JBS J04 (i think thats what they're calling it now) turbo upgrade. Again you'll need the intercooler/downpipe/cat etc etc.. but you're knocking on the door of 290-300 bhp. Things start to get lairy then though and you'll almost definitely want bigger brakes.
You can get in touch with JBS on 01246 455 005.
Hope that helps.
