it's hard to tell from the marketing materials which units support what you want... your best bet is to pick up a unit that is LMT (lifetime maps and traffic). This will let you install the Garmin Mapsource software on your PC and then create maps that you can load to the unit. BUT, you must find a unit that supports routes, and THEN figure out how the hell to get it to load them. My 755 will do it, but it's a pain. My Montana will do it and do so easily, but it's expensive (I wind up with my GPS toys through bartering with a friend) and doesn't include lifetime maps. The nuvi 14xx series SHOULD all support custom routes and their are some bargains out there.
Here's some info on creating routes: http://www.advrider.com/forums/showthread.php?t=413519
Do NOT under any circumstances view the entry page for that web site or you will go buy a dual-sport motorcycle, quit your job and find yourself covered in dirt. Not necessarily in that order.