would a rim that is 5x4.5 & 5x100 with a +42 offset fit my 2000 impreza rs.

5x100 yes, the 42 offset, probably wil rub in the rear...stock is 53 and I would stay in that area.

Not knowing the diameter or width of the wheel, it's hard to tell. McDade is correct of the offset - though it might work fine depending on the tire size you choose and your willingness to roll the fenders.