Hi there lady fern and welcome to the NH forum
Just to clarify, the region you're looking into is commonly referred to as the Dartmouth-Lake Sunapee Region (the Lakes Region is further to the east). The towns along the Connecticut River in this part of the state are also collectively called "the Upper Valley".
Almost every town is this area has rolling hills (including Sullivan County), but the real question is to what extent you want to really see this topography. Since New Hampshire is so densely forested, there are relatively few places that offer sweeping vistas of the hills. Unless you find a house at a higher elevation with a yard that has been cleared for a view, most of the expansive views will be in areas with sizable farmland. Any of the towns immediately along the Connecticut River will have the most farmland, but since the valley is also relatively flat, the hills will be in the distance.
Have you looked into any towns on the Vermont side of the river? Believe it or not, the topography changes dramatically across the state line. Vermont has a lot more of what one would typically consider "rolling" hills, that is, hills that are densely packed together. Also, there is considerably more farmland in Vermont than in NH, which means a higher availability of good views.
I'm not sure about flooding in Canaan, but there's no way that the entire town is in a flood plain. Most of it is fairly elevated, in fact. Besides, flooding in NH is uncommon and extremely localized. We can't generalize about entire towns; only about individual properties that sit adjacent to rivers.
Canaan is a small, rural town (I'll affectionately call it "backwoods"), a bit more conservative than the other towns you're considering. Hanover is very much a college town, and I think the "elitist" perception comes from the fact that Dartmouth is an Ivy League school and does attract a population that is more wealthy (and occasionally snobby) than many of the surrounding towns. However, I don't want to blow this perception out of proportion, because I *don't* think that Hanover is a place where you really have to worry about being judged on your purse or your ski attire. As monied and intellectual as it may be, Hanover is still a fairly "crunchy granola" place where people appreciate the simple, down-to-earth rural New England lifestyle, and aren't likely to badmouth you based on what you're wearing. You'll see plenty of hairy, sweaty hikers trekking across town on the Appalachian Trail.
Still, if you want to avoid any traces of yuppification you'd be better off heading east; Enfield and Canaan are definitely more traditional working/lower-middle class towns. Lyme is still very quaint and rural but is principally a bedroom community for Hanover and has similar demographics. The Dartmouth influence is still present but weaker in Orford and Plainfield, which are also very rural.
The best way to make a decision is to visit and get a feel of the towns for yourself.