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.

If the price is right, and parts are easy to find while not being a rip off, they’re OK. You just have to ensure the machines don’t have design problems that will lead to quick failures such as poor thermal management.

Depends. I have my private mail system working and for the most part it works fine. However, its indeed a bitch to deal with blacklists and ISP policies. Took me two years to convince the ISP to give me a business line with two IP addresses and no port filtering. The mail system has to be configured correctly.

Knowing I have full control over everything is great, but its not for everybody.