My other car is a MINI!!!!
turbo revs / TPB downpipes are much the same thing mate, the generic ko3/s downpipes for around £100 seem to be very good and almost everyone on these turbos uses one.
The downpipes go from 3'' -> 2.5'' -> 2.25'' to meet the stock system.
Your milltek will be 2.5''. the best thing you can do is buy a 500mm length of 2.5'' stainless pipe with an expansion on one end, and cut the 2.25'' bit off the decat and slip the new section of 2.5 on, then offer it all up under the car and work out what length to cut off the new 2.5'' pipe.
Trim that down, and bolt it all on, and your miltek will bolt straight on without and reducers or other ********.
It's preferable to cut the 2.25'' section out totally rather than just expand up again to the cat back, I ran a 2.5'' cat back system for a while with the 2.25'' section in the decat, and from the colour of the steel you could see it was a noticable hotspot, going a much darker purple colour than the rest of the decat section.
No idea on coolant, I just ask for G12 / VAH coolant at GSF or TPS