Welcome.
I'm no expert, but do some research on the models you're considering before purchase.
Consider:- known problems, parts availability, class eligibility/competitiveness (if this concerns you.)
I personally consider sentimentality a big factor.
I bought a 1981 RM125X as I raced one in 1981. Not the most competitive bike in Pre '85, but I don't mind. (I'm not the most competitive rider, in any class!
Plenty of expertise on this site.
Whether you have a bike or not, try and make "Classsic Dirt", June 11/12, at Conondale (West of Malany, Sunshine Coast). A great non competitive vintage weekend.