Hi FS and welcome, Ive got a DT even older, its a 1979 DT175MX, and guess what, yes it got autolube just like the DT125R, its done around 37000 miles and the autolube has never let me down once, you need to make sure that the one on the bike you buy is functioning correctly. As for buying second hand, you could do with taking someone along with you who knows their way around a bike, someone who at a glance can tell if the chain and sprockets are worn and that the bike is apparently in good condition, bearing in mind how old and the price paid, it would be good if you could buy one from a mature owner but unlikely to find one of those in a hurry
Look out for
Receipts for work done and service history
chain wear
no rust and pitting and oil (leaking seals) on the fork stancheons, pump them up and down for a while to test
have a look round the bolt heads and engine screw heads all over bike, look for signs of damage through using the wrong tools
Generally is the chrome, paint, no dents and dings, plastics and decals in good shape
Good luck, i think its a good choice to learn a bit about bikes and have some fun