Rim diameter has nothing to do with how much you can drop a car. 17" or 18" rims all have the same outside diameter of 25.6 or so inches when you put the proper tires on. Therfore, you can go with any suspension lowering kit without worrying. Actually, when you go to something like 18" rims the lower profile tires needed make the wheel well gap look more exagerated (i.e. 4x4). Lowering the car will help diminish that look.
I have 18's and have about a 1.5" drop. There are lots of brands of springs to choose from. I do not know much about MRT, but a lot of people like Eibach springs. The Ford B, C, and F springs are also popular as well as the Steeda units. I actually have C springs in teh rear, and up front I have one coil cut off my stock springs.