If it's a good deal, buy it.
A new 3rd is only $50-100. You could swap the 5.29's over to an open carrier, stuff that in the front, and be good to go. As long as you've got it open, I'd recommend at least an Aussie locker for a 4cyl carrier or a Lockrite for a v6 carrier.
If this were a semi-DD or trail rig, I'd recommend swapping thirds and run the welded 3rd out back. You won't enjoy welded for a DD...and lunchbox lockers are very affordable.
Used e-lockers aren't bad, but that might end up being a lot of $$ if you go buy v6 gears for your new elocker, and your previous setup was all 4cyl.