IMHO you need to purchase a machine that has a dealer, near by, that you have a good relationship with. Machines will have problems from time to time. Hopefully nothing serious. My Bernina dealer is there for me. I own 2 Berninas and can count on my dealer to help me if I have problems or the machine has problems. In the past I have owned TOL machines and had NO support from the dealers (2 different dealers). Since I switched to Bernina I have had only success, both with my machines and my projects.
I have the large harp on my 830 and LOVE it. If you are going to do a lot of FMQ the large harp is wonderful.