Thanks for the thoughts chaps - invaluable as ever...
Hadn't considered memory size TBH Rob - I suspect I would get away with loading only the maps I required as & when. I have a large portable hard drive which currently has the maps on instead of clogging up my hard drive(s) so shouldn't be a problem.
Re the screen size, I am used to navigation (ok on roads!!!) on a 4½" Garmin Nuvii screen so 8.9" will be luxurious but a 10" screen would certainly be better. I don't want anything too big in the cockpit - I was/am hoping to build a carputer system around a 10" touchscreen but was considering a notepad as a forerunner to this as a trial...
Its really finding sommat cheaper than chips for now ;-)