When was the engine last rebuilt? They don’t last as long as regular B-series, some parts are rare, and a rebuild can be over £5000.
Listen for rumbles from the bottom end and look for substantial oil leaks. Oil pressure should read 50-60psi when hot.
Body panel alignment provides big clues as to what a car is really like and has been through.
The crucial places to check for rust are sills and door posts, closely followed by the flanges where the front wings attach. The chassis rails where the wooden floor is fixed are also vulnerable.
Body panels are expensive, although the doors, bonnet and bootlid are aluminium (but can corrode where they meet steel). Think about £945 for a front wing and £885 for a bonnet.