Correct Plating Process for specific applications

I would like to start plating steel parts I fabricate that end up in both a Highway and Agricultural environment. My goal is to provide better weathering, corrosion protection and wearability than what I can provide by painting them. With all the different types of plating available, how do I determine which one is the best for my application? I would like long term appearance as these are both functional and cosmetic parts. Tolerance is important because I one of the parts is a shaft assembly that fits into a bearing.

For all that you are asking, electroless nickel is the best bet. Low phos will give a better appearance and higher hardness, higher phos more corrosion protection. Especially due to tight tolerances, EN plating thicknesses are even (not varying due to current) and readily controlled.