I’ll try to help you though I use 3ds Max and not Maya, so some of my thoughts might be wrong…okay?
Go ahead and accept the fact that you will totally screw this first model up, so don’t stress out and don’t spend forever sculpting away on something that more than likely will be a learning experience. 
Yes, I like modeling in Max/Maya before bring the obj into ZB. how much modeling? Depends on what I am going for. Only working and trying it out will teach you. Learn what happens to obj’s when you sub-divide with smoothing on and off. So model a simple geometry in Maya and then import to ZB and play with the sub-divide options, you will see what I am talking about. I would normally sub-divide a couple of times with smoothing off and then turn it on for further sub-d’s. If your mesh has some density (polys) to it first it will smooth less.
I can’t answer your topolgy question because I get it correct in Max first before bringing it into ZB. I believe you would have to transfer the new topology to the old because you are screwing with the obj form. So whatever form you did high poly sculpting on you would want to have that same form for low poly so your normal and displacement maps would show correctly. Should recheck this with a topology guru though.
Sculpt away with as many sub-d’s as your system will let you get away with or when you achieve the desired results you are looking for. Once sculpting is done then you will want to create normal and displacement maps.
You can then take the normal and displacement maps into Maya and apply them to your low poly obj.
Normal maps aren’t necessarly dependent upon mesh density (poly count) but displacement maps are. So if you create a displacement map from a bizillion poly obj in ZB it will not transfer correctly to a 7 poly obj in Maya. Again, trial and error will teach you what you can get away with and what you can’t.
You will also have to have your obj UV mapped with no overlapping uv’s. You can apply uv’s in ZB or Maya. I don’t use ZB for my mapping but that is a personal preference and not a knock against ZB in any way.
I think probably your best bet would be to create a simple wall (box) in Maya and bring that into ZB and then sub-divide the wall. Now with enough sub’d’s applied start sculpting away and try to make a cement wall with a section crumbling away and exposing some rebar. Then do some searches for creating normal and displacement maps and try bringing all of that into Maya and you will see the results for yourself…what is working and what isn’t. Again, don’t spend forever sculpting because it will no doubt be wasting upon learning what works and what doesn’t. If you are trying to keep your poly count really low then you will probably have to do with just a normal and diffuse map…but that isn’t a bad thing. You can achieve some great results with just a diffuse, normal and specular map.
This is a very general outline for you and is not meant in any way to be comprehensive. This is a new toy and you will have to fall before you can run.
Good luck! 