Help with buying decision on the 510

I'm trying to make a choice on moving to DSLR from my FZ30. I can get a 510 from a BB at the closeout price, my question is in the glass options. don't know enough about the 4/3 options yet, will older Oly glass work with the 4/3 cams, what the trade-offs? I'm going to need a 200-300 zoom that is pretty bright as I am buying to take pics at horse shows in indoor arrena's.

Thanks for the input.