need advice: bumpers, mouldings, bondo, etc.

I''m hoping to paint my 92 accord within the next month or so. however, I need a push in the right direction and some reassurance that my vision is possible.
First, I want to rid of the door mouldings on the front and rear doors. My plan was to fill these in, however a friend of mine told me if I used straight bondo it would probably rot out from the inside out despite a fresh paint job overtop. (I live in Ontario and we use road salt like it''s going out of style). So for starters, I''m wondering what I can fill these gaps in with. I want the doors to look smooth all the way up and down, and I have no access to any sort of welding tools so that''s out of the question. Next, I would need to fill in the mock mouldings on the front and rear bumpers, and I know that if I bondo over plastic it will flex in the heat, correct? What could I use to smooth out the front and rear bumpers to make the car look uniform? I would like to avoid using after market bumpers as I prefer the look of the stock 92 accord. The only things I''m looking to add to the car are a front lip and a low rear spoiler when all is said and done.
 
Anyway, any help in this department would be greatly appreciated. I''m sort of a newbie when it comes to anything body related.
 
Thanks in advance!