It's hard to generalize on what it's like buying a used turbo Miata. They vary dramatically. I'll get to this particular example soon, but first, some overall comments.
Modified cars are rarely sold in peak condition. Sometimes, yes. But many times, the car's being sold because something wasn't working right and it's frustrating. So be prepared to do some troubleshooting.
Also, it seems to be common for modified cars to flip between several owners in quick succession after the original modifier sells it. Someone buys the car, discovers it needs work, isn't what they hoped or something goes wrong, then it gets passed along. This cycle repeats several times until the car's a bit haggard and nobody knows anything about it.
You also have to watch for the DIY guys. Don't get me wrong, that can work out really well. But most people who are doing a DIY setup are doing it for the first time, so they make mistakes. Reading on a forum isn't always a substitute. Meanwhile, a kit has been debugged for years by people who do this for a living. There IS a difference.
Now, this car? It's being sold by the modifier. That's good. It's apparently well tuned and in good shape. It's got quality parts with the arguable exception of the suspension, and all of it can be accounted for. It's the current generation of our turbo kits, so it shows the results of a couple of decades of evolution. The grille has to go unless you like overheating, but otherwise it would be nice as it sits.
However....
I'm not going to agree that "some Miatas have this problem" when it comes to a twist. That car's been in a wreck, I'd inspect it carefully. The frame rails will reinforce the structure, but they won't reshape it. It may just be the fenders. Ask the guy for some numbers at the pinch welds.
If it were me, I'd buy it and put it on a frame machine. The price for the car is pretty good assuming the motor pulls good compression numbers. $400-600 at a frame shop would tell you if it's twisted and probably untwist it.