Should I modify my 750dp, or just go bigger?

If I did go that route, I wouldn't be afraid to match it all up and blend it with a dremel and some sanding rolls. As long as all the internal ports lined up.

I think the one thing that keeps bouncing back and forth in my head is 1) Put $120 or so in my 750 or 2) sell the rebuilt 750 for $175 or so and put that towards a used 850, or bigger.

It would be cool to try the proform just because I have never used one before lol.