Timing chain kits can be found on eBay for around £200 in the UK. I can't speak to their quality as I am not a mechanic, a proper newbie on car DIY, and haven't changed mine yet, but reading (with some trepidation) the many responses on this, what I think I can state is that cheap tensioner kits will always let you down. Main Stealers will give you Bimmer parts for a King's ransom but adequate quality spares can be found cheaper on the W3.
As to the actual task itself - I would definitely look for Mike1967's many, many posts on here advising great care and attention (Quite a few other posters too, but from memory, Mike has responded most on this). From what Mike writes, you only get one chance to get the timing right without starting all over, and stability / immobility of the moving parts when setting the chain is absolutely critical. It is a fiddly job, I would get a trained Mini / Peugeot mechanic to do my 2010 MCS (Prince engine). And I'll probably look for recommendations on here first , as it seems to be a classical case of experience over training for these engines.
Tools - see the other posts on that, but I would want a torque wrench and the (good quality) special timing tool or I wouldn't dream of starting.
Later models - Anyone out there care to comment?