either header will work fine, 4-2-1 will probably help in the midrange whereas a 4-1 will help more on the top end. Either style should give you a little improvement over stock. Ground clearance is always an issue especially on lowered vehicles, not much you can do there except don't drive over speed bumps and steep driveways.

A 4-1 will usually hang a little lower because all 4 pipes go under the oil pan.

Clutch choice really comes down to the type of driving you do. Exedy, ACT and Clutchmaster all make very nice replacement clutches.