Hi and welcome to the forum.
I too am a newby but have learnt so much which was my main aim apart from driviing a fast sports car.
First if your a newby, like I was, then this is not a nut and bolt job kit car. You will not get everything in the kit and you just bolt together like a Westfield. It is also a challenging build as you will learn skills your not even aware of yet.
Your questions....
1) £350 for all the Rover parts needed is good money. Saving on the donor. The purpose of the donor car has two functions. 1. To supply the parts you need cheaply. But if your getting the parts new (which will cost more) then I not think a donor is necessary. 2. As a skip to pinch bits from instead of going to the scrapies.
2) Not sure which bits you will need for the honda. Would assume running gear and engine electrics. Perhaps someone wlse can add. The Stainless Steel exhaust is supplied by Marlin to fit.
3) All part of the fun doing a kit car. Individual. I think there are 4 turbo's that I know of, and all done the electrics different! There will be no car identical!
4) Additional to Module 1&2, I would order the ally side panels (both diagonal ones and if your not having doors, the side ones. You can get them from a local metal place, but I would suggest get them from Marlin. A little tip. Don't fit the diagonal ones until you have done the dash electrics. As I've just found out!
Hope that helps.