If you do a search at the vinyl asylum at Audioasylum you will see that the both the table and arm you have are highly regarded. I doubt if you'll experience much of an upgrade in the arm/cartridge department unless you spend well over $1000 and possibly closer to $1500. I have an FT-3 arm which is in the same league, possibly a bit better than the MMT and the only step up I'd make is probably into SME IV territory which is pricey (maybe around $1200 used?). Personally, I wouldn't be interested in buying used cartridges unless I knew the seller very well. I'd consider a used arm.
You don't say how old the setup is and the cartridge may be past its due date, so that might be worth tackling first.
All that being said, if you are playing beat up and dirty records, there aren't many tables at any price that can make them sound good. If they're just dirty, investing in some basic wet cleaning/vacuuming equipment like the entry level Nitty Gritty or KAB EV1 and some RRL fluids for a few hundred bucks is probably the way to go. If they're really scratched and/or groove damaged that isn't going to help though. There is lots of great vinyl out there, used and new, at reasonable prices if you look for it.
You don't say how old the setup is and the cartridge may be past its due date, so that might be worth tackling first.
All that being said, if you are playing beat up and dirty records, there aren't many tables at any price that can make them sound good. If they're just dirty, investing in some basic wet cleaning/vacuuming equipment like the entry level Nitty Gritty or KAB EV1 and some RRL fluids for a few hundred bucks is probably the way to go. If they're really scratched and/or groove damaged that isn't going to help though. There is lots of great vinyl out there, used and new, at reasonable prices if you look for it.