questions regarding xm & audio in 2001 E320

I'm looking at a nice off-lease 2001 E320 and had a couple of questions about the audio. I'm coming from a late-model vehicle with integrated XM.

1 - this Benz has an XM antenna, which tells me there must already be an XM tuner in the vehicle. Where is it located if installed from the factory?

2 - if i wanted to change the headunit, would I need to part with a piece of wood trim in the dash, or is it possible to pop in a new headunit and have it look ok? This has a trunk-mounted CD changer and the Bose audio package, so I'd likely keep the amp, use a compatible alpine mp3-capable changer, and new head unit w/ cd in-dash. is this possible without disrupting the integrity of the dash? i'm guessing the xm-tuner might complicate things in terms of narrowing my choices, can anyone offer feedback on how to ensure all of these pieces work well together if changing the headunit is an option?

Thanks!