Tranny coolers are pretty easy to install. If you can swap your plugs, rotor & cap - you can handle a tranny cooler install.
The general rule when selecting an appropriate size cooler is take your vehicles GVW (which for stock Xterra is plus/minus 4300lbs) multiply it by 3 and that is the size cooler you need. If you will be towing, jump to the next larger size.
I am running a stacked plate cooler rated for 19,000 GVW on my SAS'd/armored 2001. Works great and I completely bypassed the radiator cooler, no electric fan either.
Look at B&M's website. That's what I'm running.