Well, about the collision detection, it all depends on how far you want to take the engine itself. If you just want to limit movements and occupations to tiles itself(FYI(in case you didn't know yet) tiles are the parts the entire map is made up of), real collision detection won't be needed(because you can just check occupation), BUT if you are going to go with all of the possibilities of for instance warhammer, I think colision detection should be needed, because units and such can occupy parts of tiles, and can be angled etc.
But the collision detection is only of later concern, so I wouldn't worry about it too much, also, in 2d, it shouldn't be too hard.....

I would indeed start with the graphics engine, since you'll run into some problems if you don't, things like AI and a lot of rules have to do with a movement map, so it is best if you start with that part first, and then to be able to see it, you'll need a graphics engine.

Also, if you're looking for a strategy game which is like warhammer, you should take a look at the warhammer strategy game which was made a couple of years back(can't remeber it's name, though). It might give you some ideas.