If this is your first 4x4, I would say no. You should look for something that is stock, so that you can build it as your skills increase and for the the type of wheeling you like to do. I also wouldn't buy it unless you are well versed in Chevy mechanics, since that is basically what you are buying, and will need to be able to work on both rigs.
I would only buy it if he has upgraded the front a and rear axles as well(or keep that in mind when negotiating on a price), it is a professionally looking/clean install, and he has everything mapped out for you so you have something to follow when hunting down engine issues. A lot of people install/swap things w/o ever thinking about the next person to come along, making it hard for new owners to hunt down problems, as well as for the original installer when several years have past since the installation.
If you test drive it, make sure to go on a long test drive, combine lots of stop and go traffic, freeway speeds, and run around somewhere in 4 wheel drive. both high and low gears, so you can make sure that everything works, and that the engine and transmission cooling systems work correctly for a the truck both on/off road.
