I'm looking to replace my stock wheels on my 08 mini Cooper s.
I would like to purchase a set of 7.5 inche wheels using
215-45-17 tires what offset would be best used with this
wheel combination.

 

Cooper S wheels are 17x7 +48
GP2 wheels are 17x7.5 +45

You can use that as a starting point, I think. Anywhere between +40 and +45 and you should be good with that tire size. You might be able to go down to +35 with some alignment tweaks, depending on your ride height.

I like this site for wheel comparisons:
https://www.wheel-size.com/calc/

 

I want a set-up with a little lip.

 

Take a look at ROTA RB for a good deal.
ISAMIN

 

Do you have a site for them