I've no recommendations for a garage to do an engine swap, but there's a company called IVC who were just outside Leeds city centre and they've recently moved to some new premises round and about but I'm not sre where. They are VW specialists and if they don't do engine conversions then they may be able to point you in the right direction. Some of the companies that do them though are charging about 6-7k+ for them ! If you can get one done for 4-5k including engine I reckon you're doing ok.
I don't think that the weight is the problem with doing a TDI conversion, I think the problems are with the fuel lines and other bits and bobs that make it harder to do. A TDI would be a good conversion for a cab though with some nice low down torque and it would be something different. There's a member on here called A7 UFO who seems quite knowledgable about the TDI conversion if I remember rightly.
A 20vT is the obvious choice, reasonably new engine, reliable, easy to tune and lots available but you'd deffo need to upgrade your brakes if you want to stop.
Madone put these links up in the garage section so have a nosey at this thread.
http://www.golfgtiforum.co.uk/index.php?topic=26874.0 
