I need offsets on 17x9 or 9.5 and 17x11s.

I may look into 18x9s all around so offset for this too is needed.

I also need to know how low I can go without causing a problem or needing other things to be changed to compensate.

 

I've used 17x9.5's with both 49mm and 56mm offset in the front, and 17x11's with 50mm offset in the back. Some mild mod's required for the back with some cars. Factory 17x9 wheels (SS/WS6) are 55mm offset.

It doesn't matter much what the diameter of the rim is... you need the offset based on width, to center the wheel in the wheel well, and avoid interferences with suspension components.