It all depends on what you do, basically as long as you don't cut the shell about you should be ok once it is already registered and in the historic (free road tax) class. The DVLA have been having a clamp down on modified stuff though so it's better to stay within the rules.
Every Datsun rusts, it's the way they were made. What Datsun are you looking at? Don't register the engine swap. My Hillman Imp was never registered with the engine swap, never thought about it until now actually.
I like the idea of an na car which is why I was thinking the duratec in the mx5. Plus you get lsd and great handling (potentially) for about 2k. F20c is way way to expensive and that's just the engine. 13b scares me but I do love the idea of a rowdy rotary.
Am open to ideas though and will def keep you updated when I get it sorted