I'd recommend keeping an eye on Craigslist, or unixsurplus.com is also great, but they usually stock newer gear out of that price range. My NAS is a 2U Dell R5

I'd recommend keeping an eye on Craigslist, or unixsurplus.com is also great, but they usually stock newer gear out of that price range. My NAS is a 2U Dell R510 (Dual E5649, 32gb ECC memory), I think it cost me about $250 on Craigslist, and I've had zero issues. It will likely be difficult to find a machine that can take 10 drives, mine can take 8. If a machine has 3.5" SAS bays, it likely can take SATA drives as well, but be sure to do your own research. Also, in a lot of these older machines the PCIe card that connects the SAS backplane is hardware RAID only, which means you will have to find a new card, and possibly flash it with a new firmware to enable JBOD mode, so that your disks are passed directly to the host OS. I had to do this for my NAS but it wasn't difficult. One other tip I highly recommend, you can buy PCIe cards that fit one or two 2.5" SATA SSDs in. I run my TrueNAS off of a mirror of two small SATA SSDs this way, which frees up space for more hard drives in the front.