From what i found out when i was in the market for a E36 Cab, the 318's were the only models to come with NON-POWER tops. Some guys have told me that it goes down easy with one person, but i still think it'd be a pain in the ass.
As for the mileage, considering the power difference between the 6cyl. engine you're not saving much on gas. Take the time and find a 325/8 cab. The 328's had a FULL power top, as opposed to the 325's which had the power top, but you had to lock the top in place!