What kind of fabrics do you need to work with ?
What does the machine need to be capable of ?
I love my Singer 15-91, which I got for $20 and had fixed up for about... $70 (it was a while ago - I'm not counting the cleaning price since you can do that yourself pretty easily - I was just lazy) ?
It's 76 years old and just recently I used it to go through... an overcoat weight wool, a suit weight wool, and a cotton-linen at the same time. I've also tried it out on buckskin, and it was fine.
It also goes up to 30 stitches per inch, and it's little motor is as good as the modern professional ones from a sewing class I took.
It uses the same bobbins and needles made and used today (not much has changed).
It's button attachment is FANTASTIC.
However, I wouldn't recommend it for use with stretch fabrics - there IS a zig-zag attachment, but if you need to do a lot of stretch, you're better off with a machine that does it by moving the needle (the Singer 401 and 501 do that).
It's still fantastic after 76 years. I don't know how much use it got, though.
If you want a new "computerised" machine, I can't really offer any advice, sorry (I'm sure someone else can). : D