OP, you've been quiet for a bit. What have you been reading?
You should read the thread I started a bit ago. I asked similar questions, but I was much further along in my research than you were.
NB1 Build: NA to FI (not what you think!)
As a side note: I'd likely caution you to go under 250, i.e. in the 220-230 range if you want to keep things stock and safe. The fact that you're asking so many questions means that you're probably not going to be able to do something like rods without some help. If you make miata friends that are willing to help you along the way, then go for it, but I think you'd be better off getting something that you can run at 220-230 and build another block with rods and maybe pistons if you want. From what I've read, the rods are the weak point, but if I was going through the hassle of doing everything anyway, I'd likely just overbuild.
That said, as you start getting from 250->300, you're going to have to be way more invested into cooling etc since that can start to be an issue. It also really exposes any weak spots in your build. Stock 5 speed transmissions aren't going to last long at 300. And 6 speeds have meh gearing. Folks are looking for other transmission swap options, or you can do the 6 speed coupled with a 3.9 torsen or aftermarket gear ratio setup. I don't know too much about that last bit, but I know that it's going to be annoying to drive a 300 hp car with the gearing that my car has right now, which is a 5 speed with a 4.3 diff.
That rabbit hole is very expensive.
I also think that a lot of builds get abandoned because people with less know how than money get in over their head and can't solve problems that come up along the way. Frankly, I wouldn't be doing this if I didn't have help. I'm also keeping it relatively simple.
Definitely listen to what others have told you about building.
Step 1: Do any maintenance that needs to be done.
Step 2: Get the car running well on the megasquirt.
Step 3: Install larger injectors and tune and get it running well.
Step 4: Now that you know the car can idle well on stock injectors, it's time to add boost.
The process takes a few months to get to step 4. If you have all the parts, you can probably install the turbo in a weekend and have it running reasonably well in another day or so. At that point, I'd probably want to limit boost and keep tuning and adding boost a little at a time to see how things react and get a feel for driving the car.
You're on the right track and folks are being super nice to you here. Kudos for reading over the last couple of months instead of asking more and more questions!