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I often get asked to hang doors, and I always politely decline and recommend they get a professional joiner in for the reasons you’ve mentioned above. I’ve hung doors, but I don’t fancy sweating and cursing for several hours in someone else’s house for not much cash as I don’t feel reasonable charging full rates.
Nef-
First time I ever hung doors after starting out as self employed, I turned up at a revonation job to hang 8 doors. The doors were all pre finished going into old frames (pre finished doors are meant to go into new frames so no trimming is required) the finish on the doors was ebonised oak with a wax layer on top. As I arrived on site the builder who was running things took me to one side and basically said he was sorry, that he had tried to tell the clients that if they wanted those doors they'd need to replace the liners but they wouldn't listen, oh and that I should be careful as they were picky and each door cost £400. Once I started cutting down the doors I realised that the veneer was so thin that tear out was a real problem and that because of the contrast between the oak and the finish it could be seen from space! This meant I had to use a router to trim doors. Each door took me a day to hang in the end, luckily I wasn't on price and I had to pay for the first door I cut down to be replaced so only just came out of the whole experience ahead.
Bobbo
@nefarious is right hanging a door is tricky and can go wrong, even for us pros.
No because no cutting service will cut the door for you. They don't want to take responsibility for the measurements being wrong. Plus as has already been mentioned you are much better off offering the door up, marking, cutting, then repeating the process until the fit is perfect. Hanging doors is a drawn out process; after many years of doing it and many, many fuck ups I can hang 3-5 doors a day depending on the frames or liners.
As far as cutting the door down 40mm is a lot to take off a door. Most manufacturers have a maximum trim that is usually ~ 20mm. If the maximum trim is exceeded at best you will void the warranty at worst you'll wreck the door and won't be able to use it.