I have three old machines that I purchased from thrift stores/junk shops (one of them from Goodwill--my prized Pfaff 130!), and they all work beautifully. I paid less for all three of them combined than I paid for my first sewing machine 6 years ago (it was a brand-new Singer, and quite honestly, a piece of crap!
). I got rid of my "new" model machine a long time ago and have never looked back. I think it's a fine idea to look around for an old used machine. Here's my advice (though I'm no expert!
): Ask the place where you're buying it from if they will let you take it back if it doesn't work. If possible, see if they will allow you to plug it in and test it before you buy. My local Goodwill will take back electronics that do not work within the first 24 hours if you keep the receipt. Smaller thrift stores usually will not, in my experience. Take a good look at the machine before you buy it--check out the electrical cord if it's electric, see what condition the belt is in, see if it's obviously missing parts, etc etc. What I usually do after buying a "new" old machine is run it over to the local sew-n-vac and have them give it a quick look-over, & purchase the appropriate bobbins and any other parts/accessories I might need. Some old sewing machines may need a lot of work done. I have been very lucky in finding machines that are in great working condition. Part of the fun for me is hunting around for them.
Good luck, and I hope you find a great old machine! ^_^