Always helps to have a second person go with you - if you can't find a GTM person, pick someone who knows minis (or K series metro -if K3). Obvious thing to check, rusty sub frames. Check the body for damage - main tub is structural.
Like any kit car, check for brakes pulling evenly, uneven wear on the tyres, that it drives straight hands off. Let it idle for a while, then a good rev to check for oily smoke exhaust - guess you know all this?
Good to have build manual, old receipts and stuff, soft top is a real bonus (may soon be available new but over £600)
Also check log book gives GTM Rossa or K3 - you do not want to IVA it - apart from the time and effort I think that's around £400 - but you probably know that too
A rough one should be quite cheap, unless you have lots of spare time and really enjoy working on cars - buy the best you can find.
Mini/metro bits are generally cheap and readily available - GTM bits are rarer and being low volume, much more expensive, with some of the donor vehicles not so easy to find today.
Allen