The standard offset is +27 for the little 13s that come on Sprinters released here in Oz. But you need to figure out if the brake changes you have made have incresed your track at all as that will help in figuring out if you can use a larger positive offset.
If you go to a good wheel place they will be able to measure it all up and give you a good indication of what you can use. Do you best to avoid wheel spacers.