I have fixed up a Singer 201 from 1950 and it's my favorite machine to use. It's a flatbed in a small talbe and it's a joy to work on it. It makes it easy to maneuver large pices of uphostery fabric like chushion covers for chair and sofa seats. Uphostery and furnishings usually need top stitch thread or extra strength and the machine handles it fine.
Not all newer machines behaves well with heavier threads, regardless of tension setting, so you have to choose model carefully. My Bernina 730 as well as the older 530 will behave just as well as the 1970s 800 series mentioned. I have an Elna Supermatic that's suprisingly tough and relible, my point is mainly that an older machine can be every bit as good as a new one.
The difference is mostly various auto features computerised machines can add in plenty compared to mechanical machines. There are lots of nice models to choose from. The only features you will not find on the older models are 9 mm zigzag (actually I think some Japanese high shank machines from the 60 and 70 had something very close to it). Few have variable stitch width on stretchy stitches. I haven't seen one step buttonholers either, but there are buttonholer attachments with something very close to one step buttonholes. Some kind of buttonholer feature are very common though.
Even if you take out your old machine, clean, oil and sort it out, you can always go stest sew some new models. I can tell you, I was very surpiced at how well an old Bernina 730 compared to brand new top models a few years ago. The good ones are expensive and if you aren't looking for a particular feature there might not be that much to gain over a suitable vintage model.