It's common to have Cat1 all the way up to 40 or 50 hp machines.
A 6' mower will probably give your tractor a good workout. In heavy grass a 6' is a load on my L3000. Other than that it should lift fine. You'll also find most mowers will be Cat1 because they are really made for small tractors. I only have seem 1 being pulled behind a M series and that was where a man had plenty of money to throw away. I mean really, how many people mow their lawn with a 70 hp tractor.
Used or not, find out what the blade tip speed is. You need one that's 15,000 fpm or above if you want a quality cut. Most cheap mowers are slower and cut like crap. Learned that the hard way. Lots of quality mowers out there but you really get what you pay for.
Personally, I have a Bushhog brand that has been nothing but a great mower. Just finished it's 17th season and other than blades the only thing that failed has been the bushings in the wheels. I did the bearing upgrade so I don't expect any other problems with that.
My advise is start now and do brand research. Pick a few brands and study them. You have all winter so when one comes up for sale you'll already know all the specs and can make a smart purchase. It's way easier and cheaper to know what to buy than it is to search for parts and a way to fix it. Good luck, hope this helps.