I'm not sure I understand all the options for topo maps on the unit. I'm just starting to research featuures (talked to the dude at Wal-Mart....). If I understand correctly, the unit will not come preloaded with topo maps, but must be purchased as an add-on via download, cd, or card?
Most units come preloaded with base maps, fine for navigating in populated areas but only marginally good for wilderness areas. You can purchase maps but there are also free maps that you can download and install. This is one of the reasons I recommend Garmin over other manufacturers because most of the free stuff is compatible with Garmin but not so much with the others. You definitely want a unit with map capability, this is something that used to be only on more expensive units but these days even the sub $200 units can handle topo maps.
I have an older Garmin eTrex model, it's been bashed about quite a bit but it's still working fine after 4+ years. "Waterproof" is a misnomer but surviving dampness and dust are important, most Garmin units are good in this respect.
The only thing I don't like so much about the etrex series is the controls on the side, I keep my gps in my map case and it's a bit difficult to operate the controls through the case. On the other hand, front controls are easier but more likely to be accidentally activated. Neither are deal breakers.
Sensitivity can be an issue although for paddling use where you are mostly in open water almost any unit will maintain satellite contact just fine. For hiking under thick cover the more expensive GPSMap series might be a better choice but they are considerably more money.
I see the newer eTrex models have a larger screen, that's another of the drawbacks of my older unit, the screen is very small.