system
A chat on subreddit /r/pcmasterrace
human
Thank you! Where would I get used parts? And more embarrassingly, what parts? Or even simpler what part should I start with?
gpt
Ebay is your friend. Look for 100% feedback and a large nunber of it. Again, I honestly don't have the time to offer you spec ideas, but every time I've built a rig in thr last 5 years I've gone 75% or more used and never regretted it. I've gotten far better parts for thrle same as new. And since you're building it your daughter will never know. 

There are pc building vids out there that'll teach you all you need to know. Look up jayztwocents, gamers nexus, etc for channels that have done great jobs at this. 

Again, I'll allow for another to chime in on parts specifically as I'm rushed and its been a little bit since I've built so im not as familiar with whats out there today. Id suggest you go for a 3000 or even 2000 nvidia rtx series card. Get a B series motherboard (she won't need anything better), and min 16gb of ram (8gb doesn't cut it anymore). Save money on stuff like cases,, just get something cheap with good airflow. Get Noctua fans if you want it quiet. Go for a reputable company for psu, like seasonic, evga, corsair etc. Dont get a no name Chinese pos, NEVER CHEAP OUT on the psu.That should get you a good start. Use websites that'll allow you to punch in specific parts to ensure they all work together! Like pc parts picker. Etc.

But basically, bleeding tech people (like myself a couple years back) are always dumping barely used hardware to get the latest thing. Profit off that, get something a uear or less old and save big ans get her something she'll use for a few years to come. Don't buy a pre-built....