This is my first contact with you ....so be gentle!!
I am talking to the owner of a 1934 Speed 14 whose car is in very good apparent condition, but I was concerned when he said that the steering "wandered" unlike that on his Park Ward Bentley(!) where he could run straight at 80mph on the Autobahn! The Rover apparently was comfortable at 60-65 but the steering needed constant correction. This does not sound good to me.
He also thought the hydraulic brakes were "unique to this model" and he had to have wheel cylinders specially cast....can this be true?
The car has well known provenance and is not cheap, but these are points of concern as I would intend to use the car!
A general point. The 3 x SU set up only appears to give a small power improvement as far as my research shows about 54 vs 45 bhp or so? ok, 20% uplift, but from a pretty low base! On a 1200kg car at least 60 would be good. It is a long stroke engine of course, but....?