If you can, get a 3080 ti over a 3080. The 3080 ti is almost like a 3090 as to the number of CUDA cores, but with 12 GB instead of 24 GB VRAM.
As to the CPU, get an Intel 12th generation, either a 12700k or 12900k. I don't know why the store recommended older generations.
And as yepkoo suggested, go with 64 GB RAM.
Water-cooling may or may not be necessary. I have an older air-cooled system that works fine. My Dell R12 has water-cooling, but the case is fairly small. Get something with a reasonably sized case. Small cases are cute, but troublesome.
And stay away from something with RGB lighting, which makes your system look like a Xmas tree and causes unnecessary heat.
If you mention the country you are in we may be able to suggest something.
The problem with bought over built is that you have no control over the components. I bought my Dell last year because I wanted a 3090, which was impossible to get otherwise. The GPU died recently, but I had warranty and they replaced the card with no costs to me, even the shipping was included. So it is important to look at the warranty of what you buy.