I think the first thing you need to do is determine how much you plan to spend on the build from start to finish. Then determine how much searching for parts vs buying new you will do. Then build the best rig you can within your budget and resources.
Build the rig around the suspension. (preferably a link setup) Make it as light as you can, and as strong as you can. Most importantly read up on Pirate as much as you can and learn from other peoples experience, mistakes, and success'.
If you were smart you would look at someone who has a successful well built rig and copy it.
Here's some examples, why do you want a D300? Becasue it's cheap? well once you buy one, and you upgrade to a twin stick, then upgrade the inputs and outputs, and change the gears, and polish it all up, you'll realize for the same price you could have bought an atlas and and saved the labor of building the case.
How about that SBC, you going to run a carb? pane? FI? How much you plan to dump into that engine? You can get GEN III gm engines for under a grand and they will be more reliable, more powerful, and about the same price as a SBC.
Leaf springs? are you sure? Why even waste your time on a buggy if you're going to use leafs?
If you want to go cheap, build a buggy with toy drivetrain, v8's and tons get expensive quick!
Atleast thats my $.02 but what do i know?