When choosing a long arm machine the most important feature other than a stitch regulator is the height and length of the neck of the machine.
It doesn't need to be longer than you can comfortably strech your arms. I've know quilters who buy the bigest machines with a 27" neck and never use it to ful advantage because your arms can't control it past a certian length. I recommend some where between a 17" & 20" neck.
As far as height, remember that you need to roll up your quilt as you quilt it, and it dosen't take much before its touching the bottom of the neck. It doesn't matter how long it is if you can't roll your quilt up all the way. I recommend around 11" to 12" height.
A mid-range long arm sounds aout right for you. A lot of the "New" machines on the market today will not handle many of the quilts that you may want to do. And while you say that you only want to do 70" quilts, that may change.
Another thing that is important is the wheels and track system. There shoudn't be any drag at all as you move the machine around.
Good luck, and happy quilting.
Lanny