Sometimes your best choice is the one you can afford. Of course the biggest and best machine is the one to go with, but your wallet may not allow for that.
A computer that you buy off the shelf will probably need more RAM. On top of that, you probably will want to go with external storage as well because digital files can get quite big.
One way to save money will be on the editing software you choose. Did you know that DaVinci Resolve offers its editing software for free? Even iMovie can be used to cut a feature. The movie Tarnation was edited using iMovie and was made for under $500!