Good stuff, if you are getting them to fit the radio then get them to put the base unit up under the dash mate on top of the transmission tunnel up out of the way, there is room there. If you fit it under the seat then you will wreck it quickly the first time you get water in your car, and you likely will. It a little more difficult to fit there but they are fitting it so it aint your problem ;) - good call mate and will definitely look into it.
The VN airbox will be interesting to see mate, get some before and after pics and post them up. It will save you a fortune on Air Filters running cheap Holden ones. Bonus!! What size is the inlet and outlet pipes on the VN box?? you really want to make sure that your getting at least 3' in and out or it will starve you of power. - Will do on posting pics, it is apparently pretty common and I already have the details of the fitout - inlet and outlet are run from the GQ original pipes (95mm from memory) as that is defiinitely a problem - moreso on the inlet as the commo one is a smaller, rectangular inlet which will get covered and sealed.
I agree that the diff breather extensions are easier to fit on the hoist for sure but make sure they aren't robbing you for the kit mate?? at the end of the day for you to do it it will cost you less than $40 for bits but they may sell you an packaged "breather extension kit" for more than $100 and charge you another $60 to run the hoses. ask them for an idea on the cost of that little mod cause its easy as to do yourself as you say and could save you more than $100 for less than an hour of your time in the driveway.?? They're doing it for free whilst they have it (there's no such thing as a free meal, I know ... its in the pricing somewhere!!) - good bunch of blokes though, and I have had a fair bit done there already, with a bit to go, so no drama there mate.