I don't want to burst anyone's bubble, but your goals and expectations are unrealistic.

A Bushnell Elite 3200 5 x 15 x 40 is about $329 + tax - new.
A Bushnell Elite 4200 5x15x40 is about $500+

Buying a used scope is just like buying a used car or a divorced woman, you don't really know what you are going to get until you bring her home and take a real long test ride.

The 7MM demands that you put good glass on her and not some piece of junk. I had a TASCO Ruby Red scope on one of my 7MM's and it shot like a million dollars and it was only a 3 x 9 x 40 scope.
I guess I was pretty lucky that it came with the rifle or else it never would have been on there. The problem is - as you increase magnification, you also have to have some adjustments for Parallax and the cheaper scopes do not have that adjustment built in and you will find that when you shoot more then a couple hundred yards, the cheap scope just won't be as accurate as a more expensive scope.

My old suggestion was to work some overtime at work, get a second job or steal from your kids. You need to come up with another $100 smackers. Optic's should be a one time investment, just like a good hunting rifle.