Amp for stock Bose system?

Just bought a 2002 3.2 TL Type S and I love it. I'm looking for advice on whether the stock Bose speakers can handle an amp & if so what is recommended (how many watts can they handle?). Also thinking about adding a single 10" sub in the trunk, not looking for pavement-rattling bass, just clean crisp sound. Thanks for the help!

P.S. Also wondering if I upgrade the rims to 18"s if it will upset the way the engine, suspension, etc. function? It currently has 17"s on it.

18's will be find, and I wouldn't add an amp without replacing the speakers as well. Some bose speakers are 1 ohm, and if you hooked a 1 ohm speaker to almost any 2 channel amp you will blow the amp(of course there are some exceptions). I would not replace the amp without replacing the speakers if I were you.