First off, welcome Julian, you've come to the right place! This forum will be a great help to you resurrecting your +2.
1st thought- Nothing will keep the cost reasonable. No mater how "reasonable" you think it will be, at least double it. Really.
I can't answer for the others but a spyder chassis here in the US is way out of my budget. Shipping from the UK adds to the cost even if you source one from a US supplier. You wouldn't have that expense. I chose to do the AVO inspired reinforcements (search the forum for AVO) to my good condition original chassis and use the money elsewhere. As you can see in the posts above there are not many changes you need to make to the chassis for the zetec install itself. Other than the AVO mods and relieving the front cross member I only beefed up the area around the motor mounts.
Budget was the main reason for staying with the original transmission as well. Again shipping drives up the cost of almost all of the reasonable alternate transmission conversions. There aren't too many choices here in the US. We build some amazing 5 and 6 speeds, but they're huge! Again you won't have this cost.
I'm building a car to drive on the street, not a racer. A stock zetec motor with bike injection and headers should have about the same power of a good hot twin cam motor. Fresh stock brakes, fresh suspension components, stock size 13 inch tires, and not being stupid should see me through. Well, we'll have to see about that last part.
As for all the Miata parts, I think that they will be a mixed blessing. For the most part I think they will be heaver but not necessarily stronger. Only the later turbo Miatas had any horsepower. You will also have to deal with mating the zetec to the Miata drive train. If you don't use the drive train you'll have mix and match hubs, front to back.
The only other down side to it all, IMO, would be that you seem to be bent on modifying one of the best parts of the car. I doubt that a Miata enhanced suspension will come close to the performance of the stock suspension, either in handling or ride comfort. I really don't hear any of the spyder zetec owners raving about handling improvements either. They do have the advantage of being able to use bigger wheels and tires but I don't know if that's a handling plus or not. I've never driven one. I'm sure someone will chime in with an opinion.
Hope that didn't come across as negative. What it really boils down to is that it's your car and your choices. No matter which way you go have fun and see it through to the end.
...and we like lots of pictures!