Phiiiiil posted a thread on the STW forum, pre-hack, regarding a £50 SatNav he bought and then hacked into the WinCE OS.
A few people did it and got TomTom running on the thing and Memory Map too and lots of other stuff.
I think the down side was Battery life.
So, while a cheap Windows PDA combined with MM might be a tempting Idea, just make sure your PDA has a decent/enough run-time.
I have a windows phone and just run MM on it. Only thing is, I don't mount my phone to the bars, so realtime directions aren't really practical.
In fact, in my limited off-road GPS experience, having a bar mounted, display isn't always very useful. The GPS signal could be 20m out, while you're trying to find a piece of ST maybe only 18" wide.
I've an Edge 305 and use it more for pre-loaded road rides or for trakcing where I've been when I've cycled off-road.
Solo.