I would like all the fancy stitches and stuff they can do but will try to see how much can be done with different feet and attachments. I have a Jones button-holer that seems very complicated re working out how it functions but I'm sure I'll get the hang of it and I can't see how a zig-zagger attachment can work unless it is moving the material rather than the needle understand.
It depends what you want to sew what the optimum machine is.
For a lot of utility stuff including the heavy upholstery; unless you are sewing all day, every day in which case you need an industrial machine, a solid vintage machine in good working order is as good as anything, but you are restricted if it's just a straight stitch machine.
Yes, the zig-zaggers do work by moving the fabric, same with buttonholers. I have a zig-zagger somewhere but I've only tried it once and it was nowhere near as good as a proper swing needle machine. On the other hand the buttonholers are excellent once you get the hang of them.
The big restriction with straight stitch machines is when you come to neatening edges in clothes. There are the traditional/old fashioned methods that many of us more mature ladies learned at school (the boys would have learned to use a hammer and chisel or some such) like using pinking shears or two lines of stitching, but the neatest seams will be run-and-fell or French. Of course if you line everything you make there is no need to neaten seams ...
Sadly the straight stitchers don't like stretchy fabric
especially if there is a lycra content. If you want to get into making fleece jackets, jogging trousers or tee shirts using modern stretch fabrics you really need to start saving for another machine or two. Not only one that does zig-zag and other fancy stitches but also an overlocker.
ETA - the colons either side of fabric worked!