buying 19" wheels offset questions

[quietkilla6]

im looking to get 19 inch wheels and i want 8 or 8.5, what ever size is stock in the back of a z already i want to also put in the front so that when the time comes i can rotate but im not sure what offset i should look for. i want somethink light but cant afford volks (althought the GTS volks are dreamy) i really think the next best thing would be the 19" rays from the g35 but none want to sell me just the backs and really i need two people who want to sell just the back so dont think that will happen any suggestions