What is your time worth? it will be quicker to buy one all set up and ready to go.
What is the experience/education worth to you?
the actual time to change ring and pinion is 4 to 6 hours, all set up and ready to go. maybe less on certain rears, but i have only played with those that are in the car, as opposed to the removeable R&P units (which should be easier to set up)

What have you now? From what i can gather, you need 3.50 gears AND a limited slip diff (LSD) rear too? So you can get teh 3.50 NOW and later get the LSD later? Why not go look for a used 3.50 LSD now? Or a good LSD and get the 3.50 gears later (gears i believe are cheaper than the LSD part). Just thinkin outloud...have to do this on teh camaro at some point and my Linc needs better gears than 2.24...