There isn't really much that can go wrong with one of the older machines - the non electronic ones.

It can, if used for many hours a week be worn out - all the bearings etc just worn so they are loose, but you can usually tell by the sound it makes - everything clatters.

They can also have the teeth stripped off the cogs by misuse, but that becomes obvoius when they are tried out.

There is not a lot to adjust - not anything I would have thought a repairman might be needed to put right. That might be because I grew up with a sewing machine in the house, and learned to use it at an early age.

The main thing is to get the manual with the machine. That way you know that for stitch X the stitch length is set between A and C and the width is
D to G and the top thread tension H to J, so you don't try to use settings the machine can't cope with.

Actually the really main thing is to get the manual and then read it. The manufacturors do usually try to tell you how to use the machine, though you need to be able to spot the ones where a native German has translated into English from Japanese. You can usually tell if the word 'backside' appears more than once on any page. Those teaching English for translation seem to think it is funny.

Modern machines have been part of a pricing war for some time, and forcing the price down produced cheap badly made machines. It can pay to look for an older machine which is fairly heavy. There were a few lightweights made which were good quality but they so often end up on the floor that they are rarely undamaged. Even when they have a table clamp, people do not seem to use them and then look surprised when half way through sewing a hem on a curtain the whole sheebang slides sideways with the weight of the material.

Over here in the UK you can often get second hand machines in shops where new ones are sold, as they know there is a market for them,They sometimes see the same machine several times as it is traded in for a new machine as the owner gets more ambitious.

The best thing would be to see the machine actually used to sew some of the materials you want to use with it, so you can see the results and hear the machine working. The next best thing would be to find one where someone has tried it out and the pieces of cloth are still there under the foot of the machine. I have yet to hear of anyone working out how to do some sewing on a working machine and getting it onto a non working machine with both upper and lower threads in place.