1987 dakota questions

I was offered to buy a 1987 Dakota 4x4, it has the 3.9 v6, auto std. cab long bed. has about 145k miles, and seems to run fine. It is an older couple selling it and the 4x4 has not been used in quite a while, my first question is: does the 4x4 need to be used every so often or the seals dry out and it will not work? also i have heard alot of good and bad about the 3.9 motor. also it has an electronic carb and the computer controlls the timing, I know nothing about this system. any issues i should know about this system before a purchase? is a adjustable distributor available for this motor? is it possible to swap a 318 or 360 in this truck without major fab work?
Thank you for any advise and help in advance, cheers.
I currently drive a 98 ram 1500, w/ 5.2 auto. its a great truck but the only newer mopar I have ever owned, not sure about the 80's trucks.
Thanx again