Playing devils advicate,

I thought the cambelt issue was only an issue if it breaks, why not get him to replace it as part of the deal if you are that worried about it, then you will have that peace of mind at least, also not sure abotu scoobys but if a car has done 20k without a service would it really make that much difference ok it might suffer a little more wear than a normal on but since the car is probably 2k cheaper it may be worth the risk, i don't know.

Failing all of the above being total drivel, backing out would be the safer route unless you can tie up an agreeable deal or he can prove that all the information you need can be produced.

Good luck, you have to be happy with your decision.