Bike shops are asking 100+£ for a bike assemble and it might be justified but its too much for my budget and I want to DIY it.
Is that risky to do by watching youtube videos and borrowing tools from friends?
I want to install an english threaded bottom branket with a squared tappered crankset, disc brakes (mechanical), headset (threadless including cutting stem).
I second the idea of looking for a bike co-op. Or even ask at small shops if they would be interested in teaching you. I have known mechanics who would happily stay after hours to help people who wanted to learn. In either place you will have access to the right tools and a wealth of knowledge.