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).
You can do it yourself but I very much recommend to get supervision and tools from a person who knows bikes. Otherwise you might get very frustrated if things do not work out and you might even damage the bike if you assemble it wrong or in the worst case you could get hurt. Bike assembly requires certain specialized knowledge and tools, something newcomers usually do not have.