BMW 740 I or IL Purchase

I am beggining to look for a used 1997 or 1998 740 I or IL. My first question is are there any potential maintanance cost differences between the two models? I was also wondering what the price of servicing this type of vehicle would run. Lastly whether or not it is worth purchasing such a vehicle through a BMW dealer or not? I read one of the previous threads about what to look for and have made note of that thank you.

Any guidance would be appreciated. best regards.