Anything from the eearly 70's to the early 80's are tough machines. I've got 2 old singer (yeah they are door stops for now), a pfaff serger, 2 kenmore med duty (these things will go through 1/4" of leather for some reason), and a brother. the brothers are really a much lighter duty machine. Some things to look at in a used machine:
-Check the platen where the needle enters towards the bobbin area. If there are a bunch of stiple marks around the edge, there's a good chance that the walkers (the things that move the material forward per stitch length), are a little worn. Also check the armature, where the needle is held, for excessive play. A littel play is OK.. but a lot will really wreak havoc on your budget when it comes popping needles every 30 stitches.
- Check the motor strength. a stronger motor doesn't always mean that it will go through more material... but, every little bit helps. Anything over 1 amp will suffice for what you are wanting to work with. Also, hand cranks can be a lot cheaper, but they will wear you out before they wear themselves out!
- Route some thread from the spool all the way to the needle. pull on the thread from the last point before it enters the needle. while doing this, check the spool-side thread tension knob to make sure that it does work (gets harder to pull the thread as you twist clockwise; less tension counterclockwise).
Other than that, If you've got a big budget.. get a husqvarna

The computerized ones are incredible... i will have one someday, before death, just to play around with.
good luck, and let us know what you decide on... lots of good help on here for getting started.