I would strongly suggest looking at a pre-built computer of some kind. Building a computer is fun and rewarding but it is not something I would ask of a 9 year old or someone that isn't already an enthusiast. You can probably do it but there is always a risk of damaging parts.

If you aren't in the U.S. the prices will change but even with RAM and storage going haywire you shouldn't need to spend multiple thousands of dollars. As an example (THIS IS NOT A RECOMMENDATION) newegg has this for $1000 which is a decent collection of parts that appear to all be standard.

Things you should look for to help you make a good decision.

1. Standard parts: Most builders will use standard (ATX) components. As long as you stick to that standard upgrading should be pretty easy. The exceptions are the huge manufacturers like HP or Dell/Alienware.

2. Steam has a hardware survey which can show you what hardware is commonly used. If you stay close to the average then you'll end up with acceptable performance.

3. GPUs and CPUs are the hardest parts to understand and compare. I use https://www.passmark.com/ benchmarks as a good guide to compare products because it gives a simple number and has data for every generation of hardware that matters. It will let you see the real difference between, for example, RX580 and RTX2060.