Yam TW125s are awesome. I had one a few winters ago and it was loads of fun. Simple, air-cooled 2 valve 4T engine, it’d go anywhere pretty much (just not very quickly!) You can pick them up for less than £1000.
2Ts are brilliant but they’re arsehole magnets and attract thieves like nothing else. I had a WR200 (similar to the DT125, ish) stolen a couple of years ago and I won’t have another enduro type bike now because of it.
The good/bad thing with 125s is that there’ll always be a market for them because of the learner law, so whatever you buy (as long as it’s Japanese (or European to a point)) and you maintain it ok won’t ever drop in value. Chinese bikes aren’t worth looking at (yet)- all of them we had to work on were crap.
For “pootling around/scratch the itch and some green laning-trying” I’d recommend a light-ish (sub 120kg) Japanese 4T 125. Hell’s teeth, you could go green laning on a CG125 if you wanted!
A TW125, yesterday-
edit- yes, it is a fat bike…