The first thing I'll say is the following is only my own opinion and your sure to get several.
Offset: The offset of your standard sprinter wheels is more than likely to be +27 and this offset or older style RWD offset is quite hard to find in the catalogue of the modern day after market wheel maker due to the onset of the FWD vehicle and they usually use an offset +30 as a starting point.
The offset of your wheels will pretty much not be affected by how low your car is going to be but as eveything to do with the track of your car. Which I doubt you will change anyway and can't be changed more than 25mm in NSW anyway.
You can fit say 35mm offset wheels to yoour car with spacers (which are illegal in NSW and create other strains on the wheel -meets hub arrangement) which in effect reduce the amount of positive offset the wheel has. That is the mounting surface of the wheel is closer to the centre line of the wheel.
If you make big changes to your suspension you can and might be able to get away with more. Like a coil over has much smaller spring platforms and a wider rim maybe easier to squeeze in without clashing with the spring seat.
Suspension: It is good to see that you don't want to go to low as too low will mean that your handling will end up being worse than standard.