You need several tools. Setting up disc brakes poorly will result in forever noise. Setting up a headset is the most critical job in the mix. If you don't get that just right, you will have a high speed wabble that can get you hurt.

The work is not particularly hard, there is just a right way to do it all. There are lots of little tricks that each mechanic may have. And in a shop, jobs like setting up a headset are easier because the mechanic has spares to simply swap parts if they need a shorter or longer stem bolt, or a spacer that is a few millimeters taller.

I think of all the tasks as super easy, but I'm also an advanced Maker type that has built hotrods, painted cars, and worked in machine shops.

Most bikes come mostly assembled. You're paying for the peace of mind to have a solid riding experience from the start, and to know it is safe. Good tools go a long way too, and those are expensive relatively speaking.

Half a year ago I built my bike with friend who had side hustle repairing bikes.

He had nearly all tools and know how. I will advise you to find someone who will help you with that. It is not hard but for the first time it is better to have someone to show you.

I did it because I have do it all/ bikepacking bike so I want to know how to not get stuck on the side of the road 500km away from home with heavy bike (with bags). And I know how to repair it/ maintain it to get to work each day.

Now I have racked up about 4000km on it and few mishaps/breakages later I wouldn't do it any other way because I was able to fix it and get on the road again in few minutes every time.

I've completely disassembled and reassembled a bike to have the frame powder-coated, and think it's pretty easy and no big deal.

But then again, like the other guy who built hot rods etc., I'm an engineer and might be underestimating the difficulty for a normal person.

I suggest looking for a bicycle co-op where you can work on the bike yourself, but tools and volunteer assistance is available if you need it.

Purely in terms of cost, it will cost you more than 100£ in tools and effort. However if you plan to work on bikes longer term it is worth it.

You will need a crank puller, hex wrenches, bottom bracket tool adjustable wrench (potentially a cheater bar as well), headset cup remover (bear in mind there are different sizes), plus probably some oils, lubricants etc

It isn't massively hard but will take you time. If you plan to do this kind of thing more than once or twice, I strongly recommend doing so because it's super fun

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.

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.