Great post-- just what I was hoping to find! Thanks for all of the knowledge, everyone. Very helpful for someone who is A. completely lost on this subject and B. reticent about walking into a sewing shop and asking for advice on buying used/vintage as, well, they're going to want to sell me one of their new machines...
I have a couple questions, though... I'm trying to work on doing searches but it's slow going (23 pages of results just on the keyword "vintage"), so please bear with me. I haven't come across these specific questions yet.
A question is on the portability issue (I searched and didn't see anything similar). Craigslist has a lot of older machines in cabinets. That's fine and i might be able to find a place to put one... but I'd like something portable. So, for example, if I go visit my mum so we can sew together, or I take a class and want to take my own machine to learn on, I'm not lugging around a cabinet. Can machines be removed from the cabinets for travel, or are they simply... mounted in there/don't have any sort of base to sit on? (Can I even take a class with my own machine? Or will they want me to use theirs? Am I going to hear "oh, I don't know how to show you how to use such an old machine" if I have something like a Singer 99 or something?)
I'm also wondering if I might "outgrow" a straight-stitch machine. I don't plan on doing anything too complex, but there's a Singer 99k I'm looking at on Craigslist and I see it's simply straight-stitch and not even any reverse option (I don't think, from what little I can see in the pics). Will I regret not having zig-zag and reverse? It seems a lot of people swear by these machines, so maybe not... but I'd be willing to look at some of the C-list machines that look like they might be 70s-era (Kenmore, etc.) as well. (The 99k is $40. Seller says it doesn't have any attachments but it belonged to their grandmother, who took good care of it. It would be a bit of a hike for me, and very inconvenient this weekend, to go see it-- should I even bother for that price?)
Are they or are they not difficult to re-wire? It's not something I'd be confident doing myself, but still, from my limited reading it doesn't sound like it's TOO hard or should be TOO expensive. I called a repair shop today, though, and the person I spoke with told me not to even bother if the machine needs rewiring as it would require replacing the entire motor, would be prohibitively expensive, etc. But, I assume that the older Singers (99, 201, 15-91, etc.) are almost certain to need rewiring, right? And so many people seem to have them that it can't possibly be a bank-breaking or near-impossible venture...
(Grumble: oh the joys of C-list listings with next-to-no info... I want to Google the model number for reviews!! Speaking of which, has anyone ever heard of a Kenmore 1120? I see one listed, but I can find almost nothing on it, and a list of Kenmore model numbers doesn't even show that one. I thought perhaps it was a typo and it's really an 1102, but I can find next-to-nothing on those either, inc. pics so I can compare with the ad's pics. Before I contact the seller and possibly offend them by saying "uh, are you sure of what you have?" I figured I might see if anyone's heard of this model...)
Thanks for the help.

I don't even know how to evaluate the machines in all these ads I have up in my browser right now, lol. And I can't go see them all. So I'm trying to do a lot of Googling and email questions to sellers.