well take a look here, there are several options, don't fret on listed price because once you find the one you want you can find it online cheaper
I'm going to guess you could use OS-2X and SWI-CP2
you will see those on that above link, next to those part's is a link for the installation (read that document) to be sure that is the adapter you want.
reason being is if the bose amp is bad then you will want the option to just use the headunit power output
and yes most headunits have some reasonable output 50w a channel is pretty typical.
you will need a steering wheel control adapter, some of there other kits include one but maybe the kit you need
don't worry about onstar it doesn't do anything anymore so you can bypass it.