What ever you can afford.:sombrero: Your front axle choice will be dictated in large part by lug pattern and brake type unless you want to do the rear axle too. You have air brakes or hydraulic brakes and you will want something that has the same lug pattern and type as you have now. The transfercase will be a divorce mount, you could have mechanical shift linkage or air shift. You will need to get driveline work done too when you get the T-case put in. You can do the 4x4 conversion in segments though, axle and t-case don't need to be done at the same time. Start looking at medium duty truck yards and ads. Sometimes you can get the stuff pretty cheap if you shop around. I picked up a front axle and t-case from that was off a Forest Service rig for $1600. Measure your front axle spring width too, if you get parts try to get as much stuff as you can with them. Axle and driveline flanges get expensive if you buy them new.
If you ever make it up to N Ca stop by, I have a shop and like working on stuff like this. My camper project is a 72 International Loadstar 1700 4x4 Shortbus with a raised roof. It is ex-Air Force. Thats what I got the front axle for, current axle is geared at 7.17 to 1 and its hard to get parts for.