Antec Three Hundred Two with 6+5 bays ($62)

Unfortunately, the Cooler Master N400 does not fit this motherboard, even though both are ATX.
This is the most affordable chassis that fits this motherboard, with a decent amount of bays. It has 6 dedicated 3.5” bays, 2 2.5” SSD bays, and 3 flexible 5.25” bays. It also has plenty of cooling options with support of up to 4x120mm and 1x140mm fans. (one of those 120mm fans even cools the rear of the motherboard!) Overall the case is well constructed and should last a long time.

Rosewill 4U server chassis with 15 bays ($100-$150)

I recently made a short video overview of this case (not specifically for this guide, so it's a little off-topic). It's a good case, has room for plenty of hard drives and fans, too many for my liking actually. I'd personally recommend reversing the fan wall and using Arctic 120mm PWM fans there only, and removing the front fans. I'd also recommend replacing the rear fans with Arctic 80mm PWM fans. This will allow for plenty of airflow but keep the noise levels at a minimum. The chassis takes a standard ATX power supply, and supports SSI-EEB / E-ATX motherboards, but does not support larger SSI-EEB+ / EE-ATX motherboards. Rails are extra, and are not included with the chassis.
A lower price may be available from either Rosewill’s ebay store or Newegg. Amazon pricing fluctuates often.

Pairs well with another Rosewill 4U DAS chassis setup!

Western Digital 8TB My Book/Easystore/Elements - shuckable 
(price varies: $140-$160)

The 8TB My Book and EasyStore external hard drives are the most cost effective (new) high capacity storage solutions. JDM_WAAAT has a video guide detailing the shucking process without causing damage to the enclosure. If the drive inside is a White 8TB instead of a Red 8TB, you may need to perfrom a simple 3.3V mod with tape for the drive to function outside of its enclosure. These are great drives at a fantastic price.

Seagate Expansion 8TB Desktop External Hard Drive - shuckable
(price varies: $130-$150)

The Seagate Expansion drives are usually cheaper than WD’s offerings. The drive inside is shuckable, but it’s a SMR drive. These drives are great for archival storage, and situations where data is written once but read many times. (media storage, long-term file storage and archival) Write performance does suffer under some workloads, but read performance is on par with WD’s drives. I’ve seen these as low as $120 for 8TB, whereas the lowest I’ve seen a WD is around $135.

Silicon Power A55 256GB / 512GB SATA 2.5" SSD

These are fantastic SSDs for the money. Most people don't need a Samsung 860 Evo. You simply won't notice a performance difference in most applications - especially on a headless server.
 

Kingston Digital DataTraveler SE9 16GB USB 2.0 ($5)

 This is my preferred USB drive for FreeNAS, unRAID, and other bootable USB ISOs. It's metal.

Arctic 12 / Arctic 12 CO heatsink with 92mm fan ($20)

If you've spent any time around here, you've heard about the legendary Arctic 12. It’s a great heatsink, and normally it’s my go-to option, but with this build there are a few limitations. First, you can only mount it one way due to space constraints around the socket. Both heatsinks would need to have their air flowing in line, towards the top of the case in the Antec, or perpendicular to the airflow in the Rosewill. They will work fine in the Antec because it has a top exhaust fan, but I would only consider them if you can’t afford the Dynatron G785 below. Their large size might also interfere with PCI-E expansion cards, so measure first if you decide to get these. I would not recommend them at all for the Rosewill chassis.

Only recommended for use with the Antec case.

Dynatron G785 Socket 1366 Xeon 5500 2U CPU Cooler ($30)

This is my personal preferred option despite my normal preference for Arctic heatsinks. It’s $10 more expensive than the arctics, but the form factor is much more usable with this motherboard. You’re not limited with the way that you mount it, and you’re much less likely to run into space issues with expansion cards. Overall these heatsinks have a slightly smaller footprint than the ones below.

Works great with both the Rosewill or Antec cases.

SUPERMICRO SNK-P0038P CPU Heatsink ($17)

This is a great heatsink option for the Rosewill chassis. Active cooling fans are not needed with the L5600 series processors. It’s extremely affordable and keeps the noise low due to its lack of a fan.

Only works with the Rosewill chassis.

Arctic 80mm PWM PST 5 Pack ($20)

You'll need these if you want to change out the fans in the Rosewill. Even though you only need 2 for the chassis, it's always handy to have a couple extra small fans. They are useful for cooling chipsets, or Sun Oracle F40's for example.

Arctic 120mm PWM PST 5 Pack ($24)

You probably won't use all of these fans, but it's cheaper to buy 5 packs than it is to buy singles. It's also nice to have spares on hand just in case. These are PWM fans and have PWM Sharing Technology, which is nice (you can daisy chain the fans together for convenience). They are quiet and move a good amount of air. I use them in all of my builds.

It’s not necessary to purchase these for the Antec Three Hundred Two case, but if you want additional cooling or quieter operation, 1 5 pack of fans is plenty.

Monoprice 10 pack SATA cable ($14)

This motherboard only has 6 SATA ports natively, but if you add a SATA 3 card for SSD’s you could have up to 10 ports. It’s always handy to have extra cables.

4 port SATA 3 card ($30)

This is great if you want to get full speed and TRIM support out of your SSD’s. Regular HDD speeds won’t surpass SATA 2, so reserve the SATA 3 ports on this card for SSDs only. There is also a 2 port model available here for $14.50.

LSI 9201-16e ($35 OBO)

If you want to connect an additional 16 drives in a DAS, this is the way to do it. It supports IT mode only, so no need to entirely erase it and reflash. It likely will just need an update to P20. Check out the DAS guide here!