I'm doing a Rover turbo. There are three other that I know of. Brian, Trev and another John. Brian has completed, Trev and John are on there last legs to completion.
Rover or Honda. The Honda was not an option at the time I got mine. But managed to get a brand new Rover 220 Turbo engine. Got her started about a month ago and she sounds great.
You say 280 BHP from a Rover engine. But as been discussed on the Rover forum, to be reliable, you need a forged bottom end. And this is going to cost lots.
Not sure on the Honda pushing that power whether the same is recommended.
Only you can decide.
But also you have to way up the cost of getting the doror/parts etc to build.