It might help if you state whether you are looking for a concours (mint, pristine), driver (older restoration or original with a nice "patina"), or restoration project. Derek has pointed out the frame importance but if you are looking for something in the first to categories it is hard to tell if a good bondo job has been done to cover skin panel defects. Sometimes it is difficult to easily see if excessive amounts of body filler are in place and in the long run that can cover a multitude of crimes against a vehicle. Take someone who has done some British restoration work with you if you have such a contact. If you are looking for a restoration project there are certain parts that if missing a beginner may think of as minor but you will find them rarer than hens teeth and very expensive. Steering wheels, trafficator assemblies (turn signal) are among them. Obviously a complete drive train will at least give you a core that can be rebuilt. Windshield frames and posts would be hard to find if missing, complete door latches, boot lids, hoods and soft top frames might be hard to find or expensive. You can buy almost any skin panel new if you are willing to import and pay through the nose but regardless of quality (some are better than others), you will have issues making them fit your car in some cases just because I suspect the original manufacturer had to hand fit in many cases. As pointed out above the front and rear shrouds are aluminum and finding repairable used units is not impossible but sometimes even bad ones are expensive. I do not know if they are available new. I'm sure others may know that. If looking at a driver, it is a bonus if the overdrive, if so equipped, works. Most defects can be repaired but it can be a big job if a teardown is required. Same with transmissions. There are 5 speed conversions out there and many have done that for reliability. By all means pull the oil screen in the gearbox and look for coolant in the engine oil if you are able to do a prepurchase inspection. It may be worth paying for the oils to be changed if you are looking for a driver. By all means keep asking questions here. I consider myself a novice in the shadow of some of these folks.