I used to buy machines from shop goodwill. They have become way over priced and come in repair ready condition. Some require way too much repair to be worth repairing.
I have also bought and repaired machines on Craig's List. At least ther you can check them out. There are people who sell fully repaired machines on Craig's List. You might look and see how the machine is listed. If they are asking $20,000 for an old machine, they are likely drug users... Or maybe deceived at best... If the machine is listed as restored or fully serviced, look for other listings by that person. You may have found a place to buy exactly what you need, a good machine that has been put in servicable condition. That person may be able to point you to a decent machine exactly what you are looking for. When I pick a machine out of my herd to use I usually have some task in mind and some machines are better at some tasks than others. Then remember, about 99% of your sewing will be straight stitches. Some of the really old machines sew straight stitches better than anything newer. Left homing machines can be very nice to use. I have two nearly identical machines made by the same company one left and the other centered... I prefer the left homing machine over the centered. I don't know why though. There are Kenmore's I like and some that hate me.