1. A generation 10 will work fine in a standard apartment so long as the power supplies installed take the same voltage as your mains. I can't speak about the fan noise much, but there may be ILO hacks to control that. Make sure you're running compatible hardware as HPE systems tend to run much louder otherwise.
2. The power draw when idling greatly depends on the installed hardware. There is no definite answer until you can monitor its consumption from the wall.
3. A gen9 is suitable for quite a while yet, and the prices are going way down for that generation. A generation 10 would be the better buy if you don't plan to upgrade again for a while, but parts are going to be more expensive.
4. In terms of parts, Ebay is often one of the best places. Rather than looking up the machine generation/model, look up the part number which is on the component itself. It'll give you much better results not only on Ebay but Google as well.

My DL360G10 is really quiet, it has the premium fans instead of standard. I’m not sure if that makes a huge difference. It’s definitely the quieter part of my lab.

I have a DL360 G10. Same server but 1U.

1. The server is quiet as hell already. I have it in my basement though, but honestly I don't notice it next to my R720s.

2. My power draw right now is 119W. I have 6 SSDs and 12 DIMMs, with 12 VMs currently on it.

3. I use eBay exclusively tbh