upgrade to XM and Remote Start?

I'm considering a nice 2006 G6 (V-6) sedan, but it has the base radio and does not have remote start. What are the best aftermarket or offical GM options for me? For what they are asking for this car, it seems like it would be a better deal for me to buy this car and upgrade than it would be to look for one with the options I want and pay thousands more.

I'm new here, and this site looks great... If I buy, I'm sure I will be a frequent visitor! Thanks in advance for any help.