First of all you have to decide if you have room for a quilt frame. They come in all sizes from twin to king size. Mine is 12 feet long from end to end. Check out the Grace Company to see their frames and get some ideas of what is available. They have them in wood and metal. Then you have to decide how much you can afford for a machine. It has to be a machine that allows you to drop the feed dogs and it should have a stitch regulator. Also it needs to be compatible with the frame you choose.
I quilt with a standard size sewing machine so when I have a quilt loaded I only have approx. 5" of quilting space (times the length of the quilt). If this is okay with you, then you can use a standard size machine. If you want more quilting space then you need to get a machine with a longer throat (the distance from the needle to the body of the machine.)
Hope this helps!