As one of the old mini guys, I think you must just do it nicely, the bakkies are becomming very rare and cutting one will be a shame. It you do it just try not to do anything that can not be undone.
As for getting one, you will see on the prices that they are not cheap anymore.
You should with some planning get a small turbo on there without cutting the fire wall. I have seen one done with the turbo mounted above the clutch housing. But there is no way you are going to get a intercooler in there. And I think building a high reving NA motor will give you a better drive and probably more relaiable.