Or a motherboard with enough ports to make an HBA unnecessary. In your case, you can deliver the 8TB of storage you've been using with as few as four 4TB disks, which just about any off-the-shelf server will support. For best performance for block storage, you'd combine them into striped mirror vdevs (using ZFS) and utilize no more than 50% of the total capacity (so you might need more or larger disks). For better resilience, you'd use RAIDZ2. Then add plenty of RAM and you'll be all set. Depending on other factors, you might need a dedicated SLOG device too.
Since we're on the topic of purchasing hardware, you're going to need to do some thinking about what your actual needs are. From experience, it's best to get everything when the wallet is open (rather than making lots of small requests over a period of time). Some things to think about are:
1) How much RAM do you need? (this depends on workload. ZFS is a RAM-hungry monster).
2) Are you better off using file-based or block storage for your needs?
3) Do you need a dedicated SLOG device?
4) How fast does your pool need to be? (it's probably not a good idea to use a single RAIDZ vdev if your FreeNAS server is providing VM storage)
5) How much storage do you need? (ZFS likes it when your pool has lots of free space).
6) How are you backing up your data?
There are lots of other factors to consider, but that will get you started. Probably the cheapest server-grade system you can get is a Dell T20, but it may not be adequate for your needs.