If you need a simple, small straight-stitcher, 185's and 99's are great solid little machines. They're also pretty common so sometimes you can pick them up for a song. I lucked out and found a 185 at a thrift shop for $15, just needed a little cleaning up and works great. Someone else flat out gave me one of my 99's because it needed a new motor - $20 later and I have a very nice little 99!
Easy to find feet and accessories (any Singer low-shank side clamp feet will fit - very common), easy to work on, nice little machines. Easy to sew with, too.
Just be careful you don't accidentally get a 285 - they look nearly the same as the 185's but are very different underneath. They have a reputation for being loud and having a lot of vibration and are not well-loved. I haven't sewn on one personally but after seeing how they're made I don't doubt that a 99 or 185 sews a LOT smoother.