I am looking at a 2001 forester 4cyl man trans 172k miles. what should I be looking to check that is a known issue ?
You should be looking at another vehicle which is more robust as it ages, like a RAV4 or CR-V.
As already explained by other members, you’re playing with fire if you buy a 20yr old high mileage Subaru & expect it to run trouble free like a Toyota or a Honda.
No matter which car you buy, always get it inspected by a trusted mechanic, before you buy it. Especially with old high mileage vehicles. You don’t want to get stuck with something that’ll fall apart a month after you buy it.
Engine problems, the fuel system had a few, electrical and transmission. The Subaru's that old are kind of a toss up.
Head gaskets and front drive axles are common problem areas.