With vintage machines my approach is to be a bit open minded for the machines that turn up at any given time. I have a hunch of what's worth going for and at the right there's not much to risk; a good vintage machine is durable and the additional cost of handing it in for a service should be worth it. Often a good DIY clean up is all a machine needs (checking belts and bobbin tyres, even replacing the odd plastic gears on some later models). The time and effort spent on a good machine results in a very dependable and capable machine. That's why all metal models are highly regarded, as well as the cast iron old straight stitchers; along side a more light weight zigzagger they are up to any job.
When I take on a cast iron straight stitcher I expect belts and bobbin tyres need replacing, as well as checking motor and wiring. Replacing this is around $20-30 and a cople of hours work as a DIY fixup, motor brushes can be extra. Newer machines are often fine but needs a good clean up and lubrication. Sometimes all that's needed is 3/4 hour of cleaning and oiling and spending some time getting to know the machine. An all mechanical swing needle machine with lots of stitch patterns to choose from can take quite a bit of oiling, turing the mechanism, letting it seep in, more oil and test sewing; which means a few turns of the same things the next week or so. If something has caught your eye don't worry too much, it's usually well worth it. I had my old 99K in for rewiring and regreasing the motor it's the most expensive service I have had, but I don't regret it. Since then I have found the vintage sources on the web, and a DIY fixup is well within reach; still handing it in for to the right service guy has been well worth it too (I have handed my vintage Bernina in a few times).