I found an ebay trader that sells a complete dual din system that replaces the top screen panel and managed to order just the surrounding panel without the rest for USD$65 +$25 shipping.
Ebay Linky
The plan is to modify that panel to mount the nexus 7 using some magnets whilst still keeping the original little red screen mounted in behind it. I've also ordered a couple of different hard cases for the nexus 7 to mount the corresponding magnets. Not sure which one will suit best and since they're only a couple of $ each an extra isn't going to break the bank.
Spend so far (AUD):
Nexus 7 $314 (for those in the land of Oz looking for one it's from The Goodguys)
Dash Panel $89 -ish (current conversion from USD to AUD is pretty much 1:1)
2 hard cases + screen protector (Anti-Glare) + OTG cable $10
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$413 and counting.... should be able to keep it under $500.
Still need to buy magnets for mounting the nexus 7. Possibly will need some sort of wrap or need to do some fibreglass work for the panel to get rid of the sd card slots in the top. Hopefully there isnt too much modification required to get the nexus 7 to fit in there nice n snug.
Also might need to spend on a GPS app if the free ones don't work too well.
Now I just have to wait impatiently for all the bits and pieces to arrive so I can get it all modified and installed.