Floor pan / Front floor brace welding questions

Just ordered a new left front floor pan and front floor brace this week. I've drilled out the spot welds in preparation for putting the new brace and pan in this weekend. Until I pull the brace out and get a look at the bottom of the floor pan, I don't know how much (if any) rust is in there.

Questions I have are:

1) If I do have rust, will grinding out the rust get rid of all the rust? I don't have access to sand blasting equipment so I can't go that route.

2) Could I apply Rust Block (Por15-type product) into the area between the floor plan and the brace prior to welding?

3) Should I just stick with using the weld thru primer on the bottom of floor pan I?

4) Should I put anything else down between the brace and floor pan to keep rust at bay prior to spot welding the two pieces together?

5) When welding in the new floor pan section, I am undecided if I should lap weld or butt weld the pieces together. I've never welded before so this is my first attempt at it. I am concerned about rust starting in the area that is lap welded so prefer to butt weld. If I take my time, how difficult is it to butt weld the floor pan?