I went through an agonising 3/4 years with a turbocharged DC2 so know the score and my 2p's worth would be for a DC5:
1. Buy everything from America.
2. Fit it yourself. it's not rocket science.
3. Get the car taken (Don't drive it!) to a good tuning place and let them map it and make any final adjustments. I would only use Will @ RDT as my personal experience has been fantastic + his hub dyno is an exceptional piece of equipment. To this day I've never driven a car as smooth and usable as my Dc2. On the flip side I had a 241bhp EP3 previously tuned by TDi North which drove really badly - flatspots, not throttle control, almost like the ignition timing was all over the shop. Race-Tech setup and tuned by DC2 originally and they did the worst job I've ever known. (road tuned, off a base map, detonating all over the place).
4. If it costs more than £3k all in, think about what else you could buy...