I'm with dvd on the insurance issue. You need insurance before you can obtain VED for the car (Road Tax) and, although you will pay proportionally more for a Short Term Policy, it will save you money should you decide to return the car and will alleviate the pressure on you to make a wrong decision should the mechanics report be questionable or vague.

As for GAP Insurance (Guaranteed Asset Protection), this can cover various scenarios. Some will pay the difference between an insurance settlement and the original Invoice price of the car, whilst others will only insure up to the Finance Settlement value. Check that you know the exact terms of the cover. Obtain comparisons - there is a link on this site for starters - and check on-line quotes before accepting what the seller is offering. I used to buy GAP policies wholesale for a Motor Group, and, although recent threats by the FCA have improved the customer offers tremendously, there are still big differences in the value of policies offered.

Good luck with your deliberations and with your ultimate choice of actions.