It is a lot easier for me to quilt with my midarm than it was pushing a quilt through my domestic machine. If you have not mastered free motion quilting on a domestic machine, you will have a learning curve with a longarm. However, if you get one with a stitch regulator, that learning curve will be quite short.
What I did is buy a used Voyager 17 with Hinterberg stretch frame from a fellow quilt guild member who advertised her setup for sale. She wanted to upgrade to a setup with more bells and whistles, common among longarmers so there are often good used setups for sale. Mine does not have a stitch regulator (I will want to add one when I decide to do ruler work) and cost $3,000 total; however, someone on the board found a similar setup near Chicago (with a stitch regulator) for $2,400. A good used setup can be a relatively inexpensive way to get started with longarming. My 17" Voyager is technically a midarm rather than a longarm, but fine for my usage. If and when I have $15,000 or so to invest in a more technologically advanced system, I would be looking at an Innova with lightning stitch (regulator).
There are several online sites that advertise used longarms for sale. Here are two:
http://www.houseofhanson.com/formerlyowned.html
http://www.longarmuniversity.com/machines_for_sale.htm
You can also look through your local Craigslist.
You really want to be able to try out a setup before purchasing. At minimum you need a machine and compatible frame. Machines range widely in price, features, and arm length -- everything from a Juki with a 9" bed to 28"+ longarms. Frames likewise vary widely. A stitch regulator is very helpful for many people, but not strictly necessary.
Edit: If at all possible, attend large quilt shows where you can try out a variety of machines. This will help give you an idea of what is on the market new, what features are available, and costs (which can become astronomical!). I did this but did not want to spend $10,000+ on a setup. From researching on the internet I knew that a Voyager/Hinty would be sufficient for my startup needs, so when one came up at the guild I knew right away that I wanted it.