Most of the canned gps system's out there don't do topo. Used to do Search and Rescue in Seattle and its nice to have the declination lines. The best is a gps reciever mated to a laptop. Easy enough in a full size truck like I had at the time, but not very doable in the '3 if you expect to ever have anyone sit shotgun. As is you can get a marine gps from garmin that you can download the topo formats onto and works pretty good from what I understand. The downside is that its for marine purposes... good for durability etc. stuff built for boating is bomb proof, but I don't know that they do the land stuff quite as nicely as say my 2730 does. Also (I'm presuming your driving a 3) there isn't alot of inches from the top of the dash to the bottom of the rearview mirror. You can easily block out a significant percentage of your visibility from the wheel... wich is at a premium already, and offroading... yikes. I ended up compromising the topo for the traffic feature. I'm quite happy with it but here is a link to gps I was thinking for the topo... hope that helps.
http://www.garmin.com/products/gpsmap478/