You do need a special holding tool to get the pressure plate off. they aren't cheap, but i do have one. Mail me for a chat if you like
The plate options, being STM 48T is really only limited to STM's own plates. I don't believe anyone elses will fit correctly. Lifespan is really dependant upon use etc. I couldn't honestly tell you you can expect XXXX miles. If they are slipping, and there's life in the friction material, then padding the pack thickness out with thicker plain plates is an option. the pack will need measuring and then selection of new plates to get it back in spec'.
Again, mail me or call me for a chat, if you want.