Look for lots of history with the car, preferabley from a well respected establishment, check all the panel gaps are right as lots of them have been crashed now and not well repaired, check the engine and chassis numbers match the logbook as loads of ringers and stolen ones out there, then just all the normal things you would check on any car like the engine doesnt sound bad or smoke and no rot on the bodywork etc of course.

Thats just general advice for cossies in general, im sure someone who knows more about the escos specificaly than i do will answer with some specific things to look for on an escos in particular.

Most important advice of all though:
Take your time! Its getting harder to find a decent one so dont be tempted to rush into buying one, try and buy with your head not your heart, they all look fucking gorgeous and its easy to "forgive" a few faults when your knickers are moist form looking at it