a lot of it comes down to budget as per usual Stu
Ive never fancied the pda type as there not waterproof so have to be put in a bag or box to keep dry
Garmin are the ones ive had and they have ones reccomended for bikes and Ram Mounts do the fittings to set it up on the bike
Memory is going to be the biggest problem if your going to plan a route but allow for deviations
i did a rough guide for another chap a bit back and have attatched this for you to have a gander at
GARMIN QUESTS have a 256 inbuilt memory which if you were to head for the Italian border via France/Belgium/Germany/Swiss and then back through France the Quest would give you give you about a 75 mile corridor of street level mapping with a bit to spare , it will still have a base map of primary roads as well . If you just plan on blatting through an area and dont need a lot of detail just load the areas you plan on exploring ,but you need to do this via pc before ya go
give me a bell if you want any help
07909 685855