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.

It's possible that grub is being installed incorrectly. I don't have much experience with Gentoo, but I do with Linux and system recovery. If the machine is unable to find the bootloader then it's likely not installed correctly.

You can try installing another distribution such as Mint or Ubuntu and see if it boots normally. If it does, then it's indeed a grub installation/configuration problem. If not, then it's a BIOS configuration issue.

Take note that the Gen7 also does not support UEFI, that wasn't until the Gen9. So your partitioning format or set up may also(but rarely) cause problems.

If you haven't figured it out already, make sure the the RAID volume you installed to is the controller's selected boot volume. If you have multiple controllers, ensure the controller the drive is on is set as the first boot option.

If it's that old, you'll need an older browser. Internet Explorer would probably work fine. If you don't have an old computer running something like Windows XP/Vista/early builds of Windows 7, you'll need to create a virtual machine. Then sign into the dlink that way and look at the settings.