The kits fit exactly the same space that the OEM parts do. However, the most important difference is that the parts in the kits cannot corrode or seize together because of the self lubricating polymer bushings used to replace the steel bushings. If the steel OEM parts corrode and seize, the result is costly repairs to the supporting structure in the wing. 

It's just a matter of removing the doors, pressing out the steel bushings and then pressing in the replacement parts. The kits include all necessary hardware including new bolts and spacers. Where the OEM sleeve part is flanged, the space is made up with a thin washer. Installation videos are available to demonstrate kit installation. 

Longer life-time, grease-free operation, does not support corrosion, decrease in weight, self lubricates. 

You don't need to, but it's not a problem if someone doesn't know the bushings have been fitted and decides to put lubrication into the hinges. The bushings will function properly in oil or grease, but it is not necessary. 

Marsh Brothers developed a new poylmer formula specifically for aerospace which they call AeroTough™ GF, for "grease free". Lubricant is bult into the polymer, completely eliminating any need for periodic lubrication. 

Yes, one kit will do both the left and right main landing gear doors.