While the Pfaffs are beautiful machines, I'm a big fan of vintage Singers, circa 1930-1960. They too were made with all metal gears, but more importantly, were made in such large quantities that (a) parts are readily available; and (b) they set bobbin/needle standards that are still in use today amongst all kinds of brands.
If you live anywhere near a major metro area, your best bet is CL. On any given day, there are handfuls of old(er) Singers available. The model I would focus on is the Singer 66 (or the 3/4 size 99). In their day, they were made in lots of 50,000; in other words, there's still a s+!tl*ad of them floating around. They were Singer's econ models back then, so one in good working condition can be had today for around $50-100.
Here's a 99-31 for sale in SoCal for $160:
What the seller doesn't know, but will eventually find out, is these machines aren't pursued by collectors (collectors focus on the 221, 201-2, 301 & 401), so they typically don't sell for more than $75. Either offer an OBO or wait.