Assembling an AR can have its challenges, especially if you are trying to do upgrades or match parts from various makers. Since the barrel is already headspaced, it’s unlikely you can end up with any problems worse than unreliable feeding, ejection or accuracy, but I’d still watch the brass for signs of too much pleasure when you first shoot it.
It’s not rocket science, but it is possible to have problems and if you do, it can cause a lot of aggravation and money to fix it. Some specialized tools are need to do the job well, but you can get by with common tools if you don’t want to buy a barrel wrench and stock wrench, but you may end up with scratches you would not have on a factory gun. Be very careful not to cross thread the barrel nut and do not alter the surface where the rear of the barrel sits flush with the upper receiver, this changes the chamber tolerances and can be bad. All in all, if you haven’t done this and don’t have someone helping you with some experience that you trust, I’d spend the extra money. I worked in a gun store and we made good money fixing the “do it yourself”, “Money Saving” projects of our customers.
If you are looking to save money, buy an assembled lower and look around for a complete upper at a gun shop or show. People often buy or build new uppers for old guns and the old uppers sell cheap used. Just be sure the barrel isn’t shot out by someone who had a fully automatic, you want it to hold a group.