As someone else already said, the tools alone are going to cost you more than the assembly cost. The headset alone is likely to set you back at least that much; you’ll need a press to install the cups, a slide hammer to install the race (?) on the fork, a pipe cutter and deburr tool to cut the steer tube to length–assuming that it’s aluminum and not carbon–and a tool to set the star fangled nut (I don’t recall the name of the tool off the top of my head). Can you borrow the tools from friends? Maybe, but most of these aren’t tools that a typical home mechanic is going to have, because you probably aren’t going to need to set more than one or two headset cups in a decade.

Besides that, it’s def. a risk, because assembling things incorrectly can destroy expensive parts (…like the Campy crankset that I trashed when I was working in a shop…), or make the bicycle unsafe to operate.

On the other hand, if you have knowledgeable friends that have the tools, I’d strongly suggest getting their help to walk you through things. If you’re mechanically adept, most of it isn’t hard to pick up, but it’s a lot easier when someone else is helping you learn v. watching videos. And over the long run, being able to do your own work will save tons. For instance, the shop I used to work at charged $10 (£7.80) in labor to change a tube (plus the cost of the new tube; they didn’t patch anything), which is literally a 5 minute job when you know what you’re doing and have good tire levers.