Yeh the HPFP is fairly deep down and a bitch to get to. I really really wouldn't bother getting a used HPFP.
My HPFP showed absolutely no signs of dying until one day it gave up. Buying a used one is a false economy. I would definitely recommend trying to get some goodwill from BMW as, although they will refuse to admit there is a problem with HPFP's on the 335i (hahaha good one) they are very inclined to giving good will gestures on them (strange that eh?!).
I would also recommend waiting until you start generating fault codes. If you're suffering from long cranks, it won't be long until the HPFP starts giving up completely and therefore generating some codes. When you get these codes, you should take it to be repaired.
It'll be at least £300 for the pump + diagnostic charges + labour from anywhere, not just a main dealers, so you could easily say goodbye to £450 from an specialist or £700 from a dealer.
RE: your fault code reader - you'll need a good one to pick up these codes. A basic one picked up nothing on mine, a good one that the BMW Assist guy had picked up one unspecified fault code, and the main dealers one picked up everything in detail.