I think some Indys have experience doing it, and for parts i'd consider ringing Sopers of Lincoln, they renowned for good parts service and offer decent prices for forum members.
I bought my parts from German eBay, but found it later slightly cheaper from Sopers BMW.
It's 99% easy to fit yourself, just the last 1% being a massive headache.
It's not too hard it's just that each engine model has a different ECU and requires the final wire going into a specific pin in here (not too hard) but then each engine variant has its own variations for different years, each having differing pin layouts in the ECU... and the instructions BMW supply are VAGUE to say the least!
All you have to do is work out which one is relevant to your car, there's a handy HOW TO topic on here, and plenty of people that can offer advice. It's well worth the hassle to research and avoid ridiculous labour cost of a garage