Long long ago, I made friends with my local 'sew and vac' store owner. A machine repair guy will tell you what to look for and might even have trade-ins available.

I have three mechanical machines, none less than 30 years old, and my favorite is a plain old Singer Fashion Mate 252 that I've souped up a bit. I've sewn canvas with that thing, as well as silnylon.
I look for all steel guts, no electronics, and standard parts sizes so I can bolt on aftermarket specialty parts like walking feet and such. Straight, zig-zag, and blind are the most common stitches, and I like a fast machine. Watch Craigslist once in a while. Been sewing most my life and that's my .02 cents.