It's certainaly do-able, but is it worth it? For the money you'll pay for a decent engine, then the cost to have it swapped (unless you are doing it yourself?), don't forget with the extra horsies you may want the vented discs on the front to aid stopping, and maybe a new rear brake disc set up from a GTI. At the end you'll still end up with a Driver, just with a better engine, and higher insurance than a standard GTI!
I think the main problem is swapping it from carb to fuel injection, although I'm not 100% sure about doing this.
It's worth keeping the driver and saving up for a good GTI, £800 - £1000 will get you a pretty good example, the only choice then is 8v or 16v. (8v all the way!). That'd defo be the way I'd do it, in fact I did a few years back!