It is a 126, not a 124. It will not get conservatively 25 mpg; you will get about 19-20 in town and maybe 25 on the freeway. Check to see that the AC works; if it does, check to see that the defrost ducts are closed with the AC running. If you are getting cold air at the windshield, the vac pods are not working; they are a pain to fix but most people just ignore them. If it is not blowing very cold, you can check the freon in the sight glass under the hood near the driver side headlight.

Check to see that the elec windows on all four windows go up and down; do it several times. Check the electric seats. Check the door locks; if they do not work it is either a vac problem or a bad door actuator.

Drive it hard. It should have decent pickup especially after about 20mph when the turbo kicks in; it should not be a slug. The steering should be tight. Turn the radio on and see if the antenna goes up and back down when you turn it off. See if the transmission shifts smoothly; if not it could just be a vac problem which is easy or it could be much worse - it should shift well.

The dipstick oil will be dead black; that's OK - it's a diesel.

The 81 is absolutely a bulletproof car; I have one and an the original owner and drive it every day. If it is a decent car and you are willing to do a little work, you will never be sorry. That said, buy the best one you can find and stay away from rust.

Oh yeah, I think $3500 is too much for this one. For a pristine one, yes but not for this one as described.