Thanks, everyone for your input!
Sounds like the Necchi is a good machine, but it is more expensive than the Singer and I don't want a machine that is difficult or expensive to maintain. The Margaret is adorable and cheaper but I think I'd rather get a Singer.
As long as everything looks okay with the 15-90, I think I will go for it. She said I can see it tomorrow, but she didn't answer my question about whether I could test it before I buy it or not. What should I do if she says it's not a good idea to test it before it's been cleaned and oiled? Should that be a deal breaker or would she be right and I should just do a visual inspection?
The 301 sounds like a great machine for FMQ, and I would be willing to forego the 15 for it, but there are not a lot of them around. There is one 301a for sale on craigslist, but it is $150, comes in a cabinet, is 2.5 hours away and I don't have a way of picking it up. I figure I'll buy the 15-90 and if I come across a 301 that I can actually get home, I'll snatch it up and if I like it better I can sell the 15-90.
Thanks again, everyone!