What kind of muffler do i buy?

The brand is irrelevant, as most are made in the same 2 or 3 factories overseas. Just make sure the muffler bearings are stainless steel; I can't emphasize this enough. If you don't ask for stainless and pinch a few pennies, you will end up with el-cheapo steel-iron bearings, and all the potential problems that go with it (rust, pitting, etc.).