i think you should sand the area where the new metal is welded, then use a newspaper and masking tape so that the paint only gets to the repaired patch. once all clean and ready take it to a painter. but make sure it is properly masked.
i like to make a square shape with masking tape, that way the painted spot looks neater, it will also show potential buyers that only that spot was fixed, not the whole floor
for places with light surface rust, remove loose rust with a sandpaper, around 1000 grit should be okay, then clean the spot with a 10% vinegar solution, finally cover it up with rust reformer (available at metro for around 1400, one spray can will be enough). mask the spot like earlier and spray the rust reformer.
you can cover up the rust reformer with matching paint using a paint brush