the 15 is a great machine - the feed dogs drop (for free motion quilting) - at least on the 15 I have (a 15-90)
I piece on a 301, just picked up a 319w that I love already - it was desperate for a good oil bath but now runs like a trouper - it came with ALL the cams, the book, the feet a cabinet, the bench - for a whopping $25!
make sure the bobbin case is there - the 15 I got didn't have one and SOME of the 15s take a different case - not hard to find or buy but a pain to track down...
you can join some of the yahoo vintage singer lists for parts, info, help, etc if you get the machine...
it's also fair to warn you that once you start you'll discover a whole new hobby....
proud owner of:
featherweight
2 301s
306
319
328
237
15-90
66 redeye treadle
66 godzilla
185
99
28k
401
not to mention the 'new' machines and bailey midarm....