best cartridge for MMF-7, stock tonearm,below1000

I'm not sure if the shelter would be a good match on the Pro-ject arm, not enough effective mass with a cartridge that has fairly low compliance. Not sure about the other two that you mention. Here's one suggestion, there is a guy over on the Asylum that is using a Benz Glider on his MMF 7 and seems happy with it. You do need to be careful with cartridge selection, think high compliance to be safe. I owned a MMF7 for a few years and while it is a great table for the money it may not be up to enabling every cartridge to do its best. I'd be afraid that you may be disappointed after spending alot of money, just a word of caution. I used a V15VxMR and was happy with it on that table.

I believe Inscrutable has the best suggestion. The Rega arm is not a drop in replacement by any means as there is too much difference in the spindle to pivot distance. You could look for a Linn-based tonearm to upgrade with, but since you seem to want to improve things I think in the long run you will be happier swapping out the table now.