A few things to look at are these.....
What is your skill level?
Have you used a lathe before, metal or wood?
How much do you have to spend on the tool?
Remember that the tooling is going to cost WAY more than the actual tool itself.
Are you looking at only making the piston parts? Are you thinking you are never going to do anything larger? Don't just think about the "now", think about things that you may want to try, or do.
How much space do you have for the lathe, tooling, materials and everything like that?
Here is the lathe that I just bought..... Great lathe for the money, and just the right size for the airsoft parts, and the suppressors that I am planning on building.
http://www.harborfreight.com/garage-shop....the-93212.htm l
Though I would serriously spend a lot of time over on this site. Read as much as you can and do some serrious searching and review watching.
http://www.mini-lathe.com/Links.htm
The thing to remember is that when you are looking at the smaller sized lathes are generally all come from the same company. They are just painted differently and named for the company. The main difference is usually the "extras" that come with the lathe itself.
Just make sure that this is really something that you are wanting, and not something "in the moment". It really isn't something that you can just hop into. Though with some practice you will be able to get the hang of things. Just don't get discouraged if things don't go the way you expect them to.