I agree with sewing pup. Take a beginning longarm class to fully put a longarm through its paces and see for yourself if it will do what you want it to do. I have one of the best, highest-rated longarms on the market and even on this unit, diagonal quilting is not the easiest or most trouble-free thing to do.
I'm also wondering if you're aware of the costs. They are NOT inexpensive by any means.