if you've found a few faults, i'd definitely get it professionally inspected
you should be able to use the results to screw down the price more than the cost of the inspection. they're very thorough, check paint thickness around the car (for accidents), they do a balance test of the engine (compression test if you want, but i think costs extra), brakes, suspension etc.
i don't believe Stags are immobilised from factory, just central locking
an aftermarket immobiliser/alarm is probably what you want, for peace of mind as well as 2 remotes