My budget is between 2000$ and 3000$.
Wow! That's a very generous budget! Your brother is very lucky to have you. I hope he appreciates what he has, and your service (Thanks!).
What is he bringing to the table? That is, does he already have a monitor? Keyboard? Mouse? Speakers? Or does the budget include buying those too?
I agree totally with going with the local shop and custom build -
"IF" it is not too late. In addition to the advantages mentioned above, going local helps support local small businesses - always a good thing. Plus, you could actually make contact with the shop, and the actual technician too. No way could you establish such a personal report with HP, Dell, etc.
Your biggest hurdle now is time. It is really running out - especially with availabilities of desired components being delayed due to supply channel backlogs and global computer chip shortages.

In fact, I would expect, and you probably need to tell him to expect delays - into the New Year.
If this is to be a surprise, I recommend you get a parent or trusted friend to track down a local shop. If not a surprise,
task your brother to find one. But tell both (and the shop) the budget is $2000, then be ready for cost overruns. Do not say $2000 to $3000 or else costs will quickly peg out at $3K, or even higher.