So getting back to the original post and questions . . .
You may not like their advice . . . but it really holds true.
A WRX at $10K is most likely older or has had some issues . . . and even the most reliable unmodded vehicle once it starts to have some age will need a little more TLC than one bought new or need more TLC than other cars of a similar age simply because folks tend to run these cars a little harder than say a Corolla, Camry, Accord, etc.
So to echo others thoughts . . .
If you are looking for a reliable, fun and cheap car . . . I would recommend something like a Civic Si or even a Honda Accord (my 5-speed Honda Accord coupe with only a naturally aspirated 4 cylinder engine was a ton of fun to drive and was spirited).
If you are looking for reliable, cheap and a Subaru car . . . I would recommend an Impreza, Crosstrek, etc. (but start saving money towards that down payment for the WRX that you really want in a few years). For even more fun, get a manual and if you don't know how to drive one, start practicing now on a cheaper car vs. a WRX.
If you truly insist on getting a WRX and can swing it . . . I would go brand new as the loan rates are generally better and you get the peace of mind of a warranty and knowing how it was maintained from Day 1.
If you insist on getting a WRX and cannot afford a new WRX and insist on getting a used WRX . . . I would try to find an unmodded WRX owned by an older person who has kept good records on how it was maintained. I know it sounds like you could be looking for a unicorn . . . but they are out there if you search long enough.