It depends what you want to do. Just because people think it is a bad idea it doesn't mean you shouldn't do it, it just means you are better informed.
Radically changing many of the main parts of the bike will mean that you may have to SVA it and it will be re-registered on a Q plate. Main parts in life frame, engine, forks etc. normally changing one is fine. I can't remember at the moment what point it goes from being ok to having to get a SVA.
Next you have insurance. All modifications have to be advised to them whether Q plated or not.
On a SR I would go bobber. Single seat with a floating rear and drag bars.
It would be cheaper to get a whole, working, bike and get the angle grinder out than start with just a frame.