Miellasmamaxo Keep in mind that streaming setups need to be overpowered compared to what the game requires without a streaming component, since that also takes GPU and/or CPU power, depending on exactly how the player is streaming. So you might hear that you "need" a certain setup when it's not actually what you're looking for.
Photo and video editing would be done outside the game, or at least not when you're also trying to play. Video capture takes extra resources too, but not nearly at the level of streaming. And you don't need any special equipment to do either one, other than maybe some extra storage space. There are free tools available that will work on any Windows machine that isn't totally overwhelmed by the load.
Is this budget in U.S. dollars or something else? In this price range, it's likely that you wouldn't save any money by building the system yourself, although you might want to buy a standalone secondary drive and have your dad install that. (It's very simple and would only take a few minutes, not like building the whole PC.) The reason to build anyway, other than that's what some people like to do, is that you can customize the setup in a way that you simply might not see in prebuilt computers, for example some people need a high-end CPU for professional loads but would be fine with a mid-tier GPU. It sounds like that's not a particular concern for you though.
As far as the desk and chair go, I don't have any suggestions. The desk is relatively simple: get something sturdy that you don't mind looking at and that doesn't have any supports or protrusions that would be in your way. For the chair, test in person and sit on what you're considering. Even the most expensive and over-engineered chairs don't feel good to everyone.
You might find a deal on either or both from a business in your area that's moving or closing, so keep an eye out for postings to that effect. These people generally want to get something for the excess inventory and quickly, and showing up with cash on the same day gets you a lot of consideration on the price.
I'll get back to you when I have time to do more research, which should be within a day.
As for my setup, it's overpowered for Sims 4, but I will say that I never had any trouble pushing 160-170 fps even on an RTX 2070 and at a 1440p resolution... unless the weather was bad, in which case the framerates could drop into the 100 range. The card wasn't close to maxed out either, in fact half the time it wasn't even running at its maximum clock rate. That was years ago though; I haven't tested that card since maybe 2021, but Sims 4 hasn't gotten much more demanding on the GPU since then. Point is, for the game itself, almost any desktop gaming-tier card from the last few years is more than fine.