Your older style Steering Wheel buttons (with the 2 phone buttons) will work perfectly with the MFD2. I know because I have had both the CD and DVD versions in my car before I got the RNS-510, and I also have the "older" buttons. Everything worked perfectly.
As has already been aluded to though, If you are upgrading from a standard unit (say RCD300 or RCD500) then the Navigation module in the Can Gateway won't be active. If you don't have highline it's not really an issue, but if you do have highline and want to see the compass, navi symbols and track details etc in the dash, then this has to be activated first it's module [37].
Might have an issue with the Iphone 3GS and charging as Apple changed the charge voltage from the older Ipods/ 1st Generation Ipod Touch, to the later Iphone units. I had the genuine VAG Ipod unit in the centre console and my 5th Gen Ipod worked and charged no problem, as did my Ipod touch (1st Gen), but when I connected my Iphone is said something along the lines of "Charging of this devlcie is not supported by this item" (or something like that).
I'm sure some techie on the forum has a solution though.
Just be aware of a couple of things with the MFD2 units. Firstly, you always have to have the map disc loaded in the unit to perform Navigation, so unless you do have a 6 disc changer or rig up a working ipod/iphone adaptor, you will only have the option of listening to the Radio whilst navigating. Secondly, no track names or indeed any ID3 information will be displayed either on the MFD2 screen or on the highline dash (no matter what Ipod/Iphone you have).