hi! help with svx problems..runs good, flexplate issue, ac

hi. nice site! I searched but I'd like more info. My mechanic says that my flexplate is cracked, and it makes a rattling noise whenever I get on teh gas. sometimes it rattles on idle. If it rattles while in park, does that mean it is not the flexplate? ie, if I put it in park, should the flexplate be spinning if the engine is on? Anyway, sounds like it's pretty sure that it is the flexplate. I know that new flexplates are cheap, but the shop wants $500 to replace it. I think I'd rather do this myself, I've done lots of car stuff before. Also what I want to know is there anything else I should do while I am putting in a new flexplate? How hard is a rear mean seal? Or something about a 'bushing' that helps center the flexplate, it can get worn? How do I replace that? What part number?

Also, my AC is barely working. Any ideas? Has anyone used special 'ebay' refridgerant?

Thank you!