The warning about the H81 motherboard refers to the fact that the parts list includes a newer processor with an older motherboard. They are technically compatible, it's just that the H81 motherboard when first released did not come with full support for all future processors. A BIOS update is needed for the motherboard to accept all compatible processors that would be released in the future. In all likelihood, H81 motherboards sold today should come from the factory with a recent enough BIOS version to support the processor I suggested for you unless it has been super old stock that has been sitting on the shelves since before the middle of 2014.
If you want to be sure it will be compatible, you can contact the retailer and ask them what BIOS revision the motherboard is shipping with.
There are no downsides to the Silverstone ML04 case, it will be compatible with and fit all of the parts in that part list.
Tell me if what I am about to say is correct: You're building a new HTPC so that will need a new Windows license of it's own. Then you have the current PC which already has Windows 7 which you can upgrade to Windows 10 for free, and then you have the gaming PC that you last talked about in August? That one will also need a new Windows license of its own. It seems like you will need two new licenses and you can just keep using your existing Windows 7 license (or upgrade it to Windows 10) on the PC that you are currently using.
How much do you want to spend for Windows licenses? Other than the usual way of paying $85~90 for each retail Windows licenses, there is a somewhat risky but cheap way to get cheaper licenses.
Cheap Windows licenses can be bought from
reddit's microsoftsoftwareswap for cheap, around $20 USD or less. Windows 10 licenses are also available for $30~40. These are most likely legitimate keys that are resold from educational programs like Technet or MSDNAA/Dreamspark. However, you are dealing with a person instead of a retailer, and informal Windows keys sales are not approved by Microsoft and probably breaking some licensing agreement, but it's not illegal. The risks involved are that the person could be selling you a fake or used key, or that Microsoft may refuse to give you support and/or deactivate your license and refuse to reactivate it. That's not very likely, usually it only happens if the seller and their list of sold keys was caught. We've had people here using those keys without issues for a long time and others who say Microsoft deactivated their key after several months. These keys should be upgradeable to Windows 10, but confirm with the seller to be sure, of course.