From what you have said, it sounds like a better way to go might be to replace that top edge of the brass outlet pipe. They sell kits in the store to replace the flapper valve and the seat (which is what you are struggling with). Have you looked into one of those? Something like this:
http://www.amazon.com/Lavelle-2003BP-Flush-Valve-Repair/dp/B001ARJR8S
Trying to sand the existing seat down, in place, until you get a good seal sounds like a lot of work with little chance of success. With a kit like this, you seal a new plastic seat down on top of the old one with a silicone sealant, then the flapper seals against the new seat. If one of those seat replacement kits will adapt to your toilet, that is your best bet, in my opinion.
Hope this helps.
There are some articles that speak to the minor differences between a class 5 and 6 toilet. The most important thing to know is that a class 5 toilet has a larger diameter trapway and the interior of the trapway is polished like the inside of the bowl is. They also use a faster low water volume flush system. The common complaint is that solids can leave 'skid marks" on the bowl as there is not a large volume of water in the bowl after flushing. the class 6 uses the same flush technology, but has more rim water action to wash the sides of the bowl better. Most reviews I've read say there really isn't much difference at all.
The better units are the Toto Drake series, the Kohler Cimmeron and American Standard Cadet series. They all can be purchased for between 200 to 300 dollars. Round bowl standard height is cheapest, elongated bowl comfort height being about 30 to 40 dollars more.
Even though we aren't suppose to endorse brand names, I will say, I have installed a couple dozen Kohler Cimmerons in private homes and in my apartment buildings. I have had a few complaints about the "skid marks", lol, but have never had a call about a clogged toilet. These things flush like a tidal wave.
The other feature I really like is that the tank always is at least half full. With average use, the water comes up to room temp and a flush only introduces 1.3 gals of new colder water. The result is no condensation on the tank during the hot humid months of summer or when the bathroom becomes a sauna after a long hot shower.
I have also installed the more expensive (under $400) Kohler Cimmeron one piece, elongated, comfort height in my own home. Looks good and works great if your looking for that low tank modern style.
Best Answer
Epoxy putty, found in the plumbing repair section or adhesives section of your local big box store:
It's a two part compound that you work together like clay. Blend a small amount, per instructions, then force it into the crack, ensuring it completely fills the crack.