I paid 4k for my MKII RS2000 , it is a good clean solid honest car , 46000 miles , 3 owners last owner had it 23 years , not a lot of history (the chap who owned it had his own garage) not a concours car but clean ,

pay less than 3k and i would say you are buying a car that will need work but will be clean (ish)

5-6k and you are looking towards the lower end concours cars