I've had a couple of Hardbodies over the years. They are nice trucks.
The only criticism I have about the Z24 engine is that I've seen a ton of them in the junkyard with thrown rods. They have long, thin rods and a long stroke. Respect the redline. I get the feeling a lot of people spin them too fast leading to rod failure.
They are pretty torquey though, so you don't have to spin them that fast to move the truck along quickly.
The Z24 is indeed based off of the L series. The block is taller than any of the L series blocks, and the bottom end of the block is a little different to make room for the longer stroke crank, but other than that they are very similar. The Z22 block with an L20b head is a popular easy swap on 510's and such and don't require much modification to put together.
The L series head on a Z24 is complicated by the taller block and the lack of an L series compatible front timing cover for the taller block, but it is technically possible to put on together with a little creativity.
I've seen a couple of D21s lately that had decent looking bodies on them, but the frames were completely rotted out. So look for rust everywhere before you buy it. My friend bought one with a really nice body, but we were working on it one day and I noticed that his frame was completely rusted out. We thought he just had a broken bed mount making the bed sag in relation to the cab, but it was actually that the frame was so rusted it was bending and sagging in the middle.
So get a good look at the inside and top of the frame rails.