If you want to cut circles in ¾" or thicker MDF, a plunge router with a circle cutting jig is the best way to go. Rotozips don't have enough power. Hole saws that attach to drills are very slow in MDF, and in something ¾" thick, a hand-held drill may not remain vertical, you would be better off with a drill press.
That DeWalt plunge router you linked is a very good one. If you use a ¼" spiral upcut bit and attach a shop vac to the port built into that router, you will collect nearly all the chips and dust you make. MDF dust is nasty to breathe.