For the dependency, it depends. Usually only "Void (Mod)" can be fine. If you notice that you want doodads that are taken in picture but does not appear in your list when searching for them it might be because you need "Void (Campaign)". The Nova ops dependency can be interesting as well as it contains quite a couple nice looking doodads. The warcraft 3 mod is not necessairy except if you wish to use stuff from that game, but there is way enough to explore within the sc2 doodads and they are usually looking better than the Wc3 arts.
About bridges, there are some doodads that can do just that and have pathing on them, but it can be tricky to place between 2 cliffs and can sometimes have some issues near the edges where a unit would look like falling far down and then come back up while moving across or entering it. Note as well there is no multi level pathing in sc2. You cannot have a bridge over another bridge for example, or passing under a bridge.
There are different way of creating a bridge.
1. Using doodads.
Pro: Easy to setup, looks great
Con: Changing cliff heigh can be buggy and require painting pathing, a bridge above lowest ground level will not work great, not all doodads has pathing on them (although you can use one of the new doodads that is a plane with pathing so any sort of doodads can look like a bridge even a fallen tree or clouds)
2. Using terrain and hide cells.
Pro: Works great whatever the cliff heigh
Con: Trickier to setup, must hide cells and thus must hide the holes that it creates in the wall with other doodads, the doodads used can have footprint that you must remove in the data, it's much harder to make it look good.
3. Using terrain and textures.
Pro: Easy to setup, efficient, looks great
Con: You can see the cliff's wall, you can always hide it behind other doodads or try to place something to make it look black... Up to you.
After that it depends how much time you want to invest, what you want to do, what you consider great / good looking, and you can trick around by using water, fog, and other stuff like that to hide parts of the terrain that looks bad. (Picture 4)