Let me refer you to an old, but picture-loaded post ...
Kenwood multimedia install - pics, pics, and more pics
Your stereo shop is correct - you have to modify the air conditioner housing to get the 911 to sit back further - not something I'd recommend for the timid. I spent several days customizing everything to get the 911 to mount just right.
It'll fit fine in the stock climate control position. I don't think you've got room for the 911 and a 1/2 Din component in the middle part - but I haven't actually measured it.
One point I'd like to mention about mounting it lower is that you'll have to pull the stick-shift back for the display to eject - it's not that big of a deal - but not something you like finding out 'after' the job is done.
Good luck - and show us some pics! :cheers: