I run the above site.
Finding this information is very difficult. Apparently there is a book in existence that details each chassis and which events it competed on. I've never managed to see it though. If you contact GSE,they claim that Baz Cannon has it, while Baz thinks GSE have it! I've not spoken to him lately though.
Several of the cars that survive have been altered now, so buying a fully original car is very difficult.
As a very loose guide:
If it has standard(ish) arches, it should be on the standard inlet and exhaust manifolds, with either an Xtrac or Quaife sequential box.
If it has the modified Cosworth wings, it should have slide throttle inlet, with a larger bore exhaust manifold, Xtrac box
If it's a Maxi, it should have the roller barrel inlet, more extreme Tony Law exhaust manifold, Xtrac box and the front and rear turrets should have been modified so the top mount position is wider. The only exception to this is a couple of gravel cars that GSE built but these are very few. Only around seven genuine wide track Maxi Escorts were made, but quite a few Kit Cars have been fitted with the arches and widened without the top mount mods.
More generally, all cars should have a GSE chassis number on the base of the main hoop and/or the front right suspension turret. The cage is quite simple, with single door bars and a single diagonal in the rear. It also goes to the front turrets and there should be diagonals behind the engine. Early Group A cars used a GEMS ECU but the vast majority should be on a Pectel T6. All should have a fabricated front crossmember, TCA/comp struts, alloy front uprights and an independent rear beam. There are a few variations on each of these, particularly the rear beam. Most seem to have the simple semi-trailing arm arrangement, while some gravel cars have a rotating twist beam like some Puma S1600s. Some Maxis use a scissor type beam, but I've also seen a fully trailing arm version of the more common rear end on one or two cars.
As with the terms "Group 4" and "Group A", the word "Maxi" is thrown about a lot, but there are a lot of cars out there that aren't the real deal. Some of them are very nice and perfectly good but, if you're looking for a genuine ex-works car, a bit of caution is required.
If you are looking at a specific car, drop me a PM and I will help out as best as I can