Want to travel Belgium and The Netherlands and had a few questions

Hey r/travel! My wife and I are looking to spend three weeks in Belgium and The Netherlands next spring. We usually do trips to Europe in three-week chunks. I don't know a whole lot about the area. I know a few places I want to go: Amsterdam, Bruge, Brussles, and Ghent. But other than the cities and for me personally (not her so much) any art museums in the area, we aren't sure what else there is to see. We are going to buy the Rick Steves' book on the area in May when it comes out, so hopefully that will give us some ideas.

Interests:

1.) We are both interested in history. I love Medieval history, and she likes Roman stuff. I'm pretty sure I'll be able to find some Medieval stuff, but my Roman history is a bit fuzzy, and I can't remember if the Romans ever ventured as far north as the low countries. Any hope she'll see a roman fort foundation, or wall? Is there anything I should look out for on the Medieval end? I know Bruges is a really old town so I have that covered.

2.) We LOVE cathedrals. We've gone to a number of churches in Britain and love touring them, and taking pictures.

3.) We love taking pictures, so finding picturesque stuff would be fun. No doubt this will happen naturally but if anyone has any specific areas, I'd love to hear about them.

4.) We both like eating in quaint fun restaurants/pubs.

5.) We dont mind bus tours if there are a decent amount of stops and the guide is knowledgeable and accurate (See: MadMax tours in SE England. http://www.madmaxtours.co.uk/

6.) I'd love to see the methods used, and results (eg. boats above where one is walking) of land reclamation.

I'm sure there are more that I'm just not thinking of just yet.

Questions:

1.) First, is three weeks too many for the area? Should we branch out into Germany or France? If one had to pick one of the two which would it be?

2.) Generally what MUST we see?

3.) Is there anything an American from the Pacific Northwest should know culturally?

4.) If we struggle with our Dutch/French how hard will traveling around be? I know on the whole most Europeans know English, but does anyone foresee troubles or difficulties for us?

5.) For those who have been to Britain, how do prices fair in Belgium and The Netherlands? I know N. Europe tends to be expensive, but. wasn't sure if this area was part of that swath.

Thanks for any answers anyone can provide!