For me, it has always been a dream. I have loved the shape and handling of the mini my whole life, but having a nice purring vtec under the hood has always been a goal.
And therein lies the rub.
The reason I stopped my conversion was that you compromise everything Mini. You are nearly always forced to run coil-overs....which are terrible. You will likely want to run larger wheels to allow bigger brakes....which are terrible.
It was just a cumulation of bad engineering all for the sake of a bit more power. It just wasn't worth it. Thankfully I didn't waste much money as I already had the engine and built the subframe myself, I'd have been very annoyed if I'd have bought one of the kits.
The other thing with the D-series engines (I was previously a Honda person, still a fan) - VTEC isn't there to provide more power, it's for fuel economy. The cams are so mild that you could lock it in 'VTEC mode' and drive it normally. It's not like the B-series (or H, K or Y for that matter) where the VTEC cam profile is quite wild (comparatively).
Anyway, that's enough from me. Good luck with the project, I hope I'm wrong and it turns out to be the car you really want. 
Nah man, all your concerns are real and I appreciate you bringing things up.
I plan on the KJB Subframe, which uses stock mini suspension.
My mini will be running 12's anyway, so no need to run bigger tires for brake upgrades.
I really want it to be as stock and clean as possible, but with a reliable engine underneath. I plan on keeping the body as much as I can keep it stock, and just cut out those inner wings if I REALLY need to. Not even wanting to box the bulkhead, I am prepared to change the intake manifold (which makes me wonder if the CRV/HRV versions are better).
In the end, I am still thinking d16. I dont even need the vtec. I could clean it up, getting it running right, and if I am craving power the smaller engine leaves me room to turbocharge.
Now if only I knew if the HRV D16 was going to be a problem.