In the interim, I highly recommend spending the cash on a decent FM modulator (in-line one so there is no FM interference). 2 minute job to install and a ten minute job if you want to "do it right" . Mine (jaycar) is better than I'd ever expected, if not kind of ugly in design. (To quote my girlfriend: "This was clearly designed by a man and not a woman." Me: "Yep. It actually works.")
That said, I took rough measurements on the depth of the head unit as that is the biggest inhibitor to a double-din install (apart from obviously cutting open the facia/HVAC to fit and making it look pretty).
The standard double-din is actually smaller in depth on the top than the bottom. This pic shows what I mean:

What I measured was what I considered a "safe" maximum depth for each tier, or in the case of a proper double-din the overall max depth based on the smallest DIN (the top).
Top: 145mm
Bottom: 160mm
Be aware that those measurements were entirely guestimated as there was no facia attached (and after more than a few cricket-inspired beers), so it was from the front of the face (facia unattached) to the rear with some "wiggle room" for cables etc. In reality it could probably be deeper than that, but I wanted a line in the sand to work out whether it was feasible in the first place. So if I find something tomorrow that is 161mm deep I'll be doing more research it it's $1000 but I'd just buy it if it was 100 bucks.