1) Blue Iris runs on Windows. So, no, it won't be as reliable as a basic NVR. But it does quite a lot that a basic NVR doesn't. The main issues I run into are video card drivers (rebooting daily can help there) and hard drive failures. Other than that, it can be a time sink if you like fiddling with things, or it can run largely ignored if you are the set and mostly forget type.
Pay for the Bleu Iris annual maintenance contract. The updates are worthwhile.
Put it on a battery backup so that it can properly shutdown if the power goes out. Power cutting out while it's recording will cause you frustration.
2) I'm not personally big on old hardware, but I am using Blue Iris in businesses, so $150/hr tech labor replacing an old failed part is far more of a concern than saving a couple hundred buying a refurb or pulling some junk out of a closet. I'd go with a current i5, 16GB memory, SSD for Windows, and the largest WD purple, red, or gold HDD you can manage for footage. They had 20TB Gold drives for $350 recently.
3) Good cooling in 1U requires loud fans. If noise is an issue, just get a regular old mid-tower. If you can let it roar (really, they get loud) then you can save space. Going with a wall mount rack might impress people that are impressed by data center stuff, but it makes maintenance more difficult.
Rack mounting is all about saving space. If you have space to spread out, the priority should be ease of maintenance. (Not posting pics of how perfectly angled your cables are online.) I also suggest using keystone patch panels, again, easier maintenance, and a desktop rack for your network gear as it tends to slide around when you move wires: https://www.amazon.com/dp/B084P2PQ3G/. Put everything else on a shelf with room to move, rotate, etc.
I mostly buy from EmpireTech Andy. https://www.amazon.com/s?me=A329YQ83EBQGJF&marketplaceID=ATVPDKIKX0DER I have had good luck with Amcrest as well.
If you are looking to save money, don't save it on running your cables. Pay a professional to do it, and use quality Cat6. You're looking at around $1,200-$1,700. Do a reasonable number of drops for other things too, like your media center. It's a long term last the life of the house investment. Yes, you can do it yourself, but it's hot physical work that benefits greatly from having the right tools and a lot of experience working with them. And can burn your house down if done without regard to fire code. Unless you have experience with it, consider hiring someone.
(Not from the low voltage wiring shorting, but from how it penetrates barriers, where it's ran, and the type of cable used.)
Servethehome did a review of an 8U free standing rack a bit ago https://www.servethehome.com/startech-rk8od-review-8u-open-frame-rack/ Looking at Amazon there is also a 4U version https://www.amazon.com/StarTech-com-Open-Frame-Desktop-Rack/dp/B084P2PQ3G/ref=sr_1_3?crid=1LR0CXA5AO7SR&keywords=free%2Bstanding%2Bnetwork%2Brack&qid=1655865047&s=electronics&sprefix=free%2Bstanding%2Bnetwork%2Brack%2Celectronics%2...
Product name: StarTech.com 4U 19" Desktop Open Frame Rack - 2 Post Free-Standing Network Rack Switch Depth for Patch Panel/Data/AV/IT/Communication/Studio/Computer Equipment 66lb Capacity w/Cage Nuts/Screws (RK4OD)
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Highest price ever: $52.41
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Previous price: $46.36
Current price: $35.99
Last price change: -23%
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The item was in stock as of 13/05/2021 18:55:20 (UTC)