Make a list of the features you need this machine to have in order to be happy.
DON'T just buy something super cheap if that's going to aggravate you when you actually go to use it.
For me, my "must have" list included the scissor cut button. I do foundation-pieced crazy quilts and that button makes my life a lot easier. I also really wanted to be able to share bobbins between my travel machine and my main machine, so I made sure they'd be compatible. I also wanted to get a machine that came with an extension bed.
My main machine is a Janome 6600, so I bought a Janome 3160 to be my travel companion as it hit 100% of my "must haves" on my list. It's a pretty great machine, it can't quite handle the heavy layers like the big machine can but for piecing and general sewing it's wonderful. It's NOT a cheap machine, but for me it was worth the expense.
Ironically, I got into vintage machines immediately after buying her, and now rotate machines I take with me on vacation so she doesn't go on as many trips now! I just need to go on MORE vacations! LOL (She's still my primary machine to take to classes, though.)