Correct workflow in Adobe Premiere for work with interlaced content
As for how to work... it all depends on what your final destination is. Are you working on a piece to be broadcasted, or for the Web? In the latter case, the final product should be progressive. If the former... the in turn it depends on your source material, which seems to be interlaced.

So you should:


Create an interlaced project, "1080i50". The "50", in this case,
doesn't mean "frames per second", but "fields per second" (2 fields
= 1 frame).
Work as you would normally in Premiere.
Export it in an interlaced format too. "1080i50" sounds good, but again, that
depends on the final destination of your piece.