Depends how mechanicaly minded you are, £1000 MX bikes are like £300 mountain bikes, probably completely borked, but possible to save them if the main mechanical bits are sound.
Had a go on a CG125 with 35000 miles on it, the myth that 4t hondas will run indefinatley is just that a myth, that thing was completley ****, no power left, clutch slipped in all the gears, etc etc. An MX/enduro bike will be even more abused.
I went to have a look at a few before the weather put paid to my motorbike ambitions before christmass. Seems its best to ask lots of questions. Buying a 15yr old 250cc crosser of a guy in his 40's who rebuilt the entire thing religiously after each race meeting for the past 10 years is an entirely different kettel of fish to buying something owned by a spotty teenager who's only experience of a big end was whilst experimenting behind the bike sheds.