first thing to look for is rust. slide the seats forward and lift the carpet inspecting the seams below the seat where the rear axle control arms mount, look under the spare tire, and look down the sides of the trunk just behind the wheel wells. If the seller will let you, bring a compression tester with you (you can pick up a cheap 'universal' kit for like 30$ at harbor freight) and do a quick compression test on the motor. All the cylinders should be within 10% or so of eachother, and this should tell you if the head gasket is in ok shape.
Depending on how much of a stickler you are for originality, the spider edition should have the same alloy wheels as the TR8s but they are silver painted, and the part number stamped in each wheel has an S at the end of them.