Only four that really work The '46 really just needs new tyres, chain and cables and the Scorpio isn't far off being put back together. I'm about ready to reassemble a "Kerrys" ladies bike that I'm building from frame up. But to answer your question... about 20 in various forms
For £400 id definitely buy second hand. You'll get loads more value for more.
Out of all the bikes I've got, my mountain bike, I hate riding that on road. I have lock out on my shocks which makes it a tonne easier but the tyres create so much drag. Some road surfaces are fine, others.... A right pain in the ass. I use 2.1" wide tyres too.
When you say off roading... How extreme are you going? If you are sticking to those choices maybe go second hand MTB with lockout on shocks and then buy a spare set of wheels. One for road, skinnier with a lower profile and then another set for off road.
I want to redo things on the mountain bike front. The only things original on the Carerra is the frame and front wheel but I want more travel in the shocks, and to go full suspension so.... that's one thing to save for.
Just cruising on it I think! Rust is staying. It's too deep in the chrome/ steel. All bearings replaced and greased, all mechanisms greased, folds fine. Brakes replaced. Cables replaced. Tyres replaced etc. Looks like it shouldn't work but it does