A sub-$700 turbo kit is a difficult thing to find. It's not even easy to build one yourself for that price. Mind you, few of his plans are going to easily come in under the $700 purchase price.
Are ebay downpipes okay? Well, they're only 30% of the equation. The design of the manifold and the choice of turbo dictates the shape of the downpipe. Make sure everything matches!
I don't consider blowing engines up as part of development. Then again, I'm lazy.
Here's the basic run down on the Hard Dog bars:
Hard Core: 1.75" tubing (as required by SCCA), sits further forward
Sport: An inch or two further back, 1.25" lower, 1.5" tubing. Why lower? Because of the shape of the roof.
The Hardtop version of the Hard Core is really just a Sport made of the fat tubing. It sits lower as well.
I'd change the order of operations. Choose your tires. Then choose your wheels. Then, if you need a widebody kit to cover these wheels and tires, get the widebody. Starting with a body kit and then trying to find rubber to fill out the body is the wrong way around.
You can do a lot better than Spec Miata suspension unless you're planning to race in Spec Miata. But given the widebody and turbo plans, I'm guessing that's not the case :) Remember, Spec cars run the suspension they do because they don't have a choice.
While Mazda Miata Performance Projects is an excellent book, (thanks for the recommendation) I think "Mazda Miata: Find It, Fix It, Trick It" might be a better choice in this case. It has more of the "why" info instead of just "how". Heck, get 'em both :)
By the way, that looks to be a 1993 in the picture. And stay away from that Dallas turbo for sale, it's for a 1.8 which will NOT fit your 1.6 engine. However, if you swap a 1.8 in to replace that high-mileage 1.6...