What are the best practices for using a caching plugin on a shared host?

I’ve launched quite a big site the other day and I’d like to incorporate a caching plugin. The setup is single-site with some Buddypress features mixed in (for user registration, maps with gpress, having a profile) running on a shared host.

My questions are:
1. Which plugin you consider best for caching given this setup and why (if possible)?
2. What are the best practices and steps I should take/consider during the setup to make sure all is working?
3. Are transients which I’ve added myself can still be used or should I remove them and leave the plugin do it’s thing? will they clash if not removed?

Thanks!

ADDITION: ‘Power’ plan on http://www.inmotionhosting.com/hostingplans.html