Sounds to me like you should consider used server hardware to stretch that budget. I'd consider the various resource guides here. For example,
Chris Moore put together an inexpensive system with lots of expansion capacity but a low entry price point. I'd marry that with a compatible server chassis from Supermicro, such as CSE-846, also purchased used. You may need a HBA or two to connect all the drives. Speaking of which, I'd go for used server-grade HDDs as well.
You don't mention what you want to use this system for, etc. so my initial suggestion would be a 6-disk Z2 array with 4 drives for data and 2 for parity, giving about 12TB of usable space with 4TB drives and 9TB with 3-TB drives (
see the ZFS calculator - FreeNAS performs best when the pool is filled less than 80%). The CSE-846 would accommodate up to 24 drives, allowing up to 4 VDEVs consisting of 6 HDDs each. With 4TB drives, you'd max out at 49TB of usable capacity (assuming 80% fill), while with 3TB drives you'd max out at about 37TB of usable capacity.
While all the drives in a specific VDEV should have the same capacity, each VDEV in a pool does not have to be. So, your initial pool could consist of a 6-drive Z2 'starter' VDEV with 3TB drives (giving you an initial 9TB usable capacity), to which you can later add another Z2, 6-drive VDEV consisting of 4TB or larger drives, bumping usable pool capacity accordingly. I'd keep each VDEV you add to a pool similar in structure to the other ones already in the pool (i.e. in this example, 6-drive Z2) for simplicity and safety.
Having a lot of spinning drives does consume a lot of power and there are tradeoffs re: number of VDEVs vs. IOPS, etc.
See FarmerPling2's excellent drive resource... to help with cost / power / etc. comparisons. I have a single Z3 VDEV in my pool here because my server merely deals with a few users and power consumption was a factor in my decision-making.
Anyhow,
GoHarddrive has some 4TB models from HGST for about $80, w/a 1-year warranty. The 3TB drives I bought years ago
are now available for $60 and feature a 3-year warranty. I'd stick to drives built for NAS use; I had no issues other than some infant-mortality with my old HGST 3TB drives.
The 3TB models I used ran pretty hot, however.
Ebay offers the CSE-846 chassis for as little as $400-450 delivered, some of them look brand new with minimal damage. The first Z2 VDEV hard drive set (6x3TB drives) would be another $380, the server motherboard, CPU that Chris mentioned would weigh in at another $500 after RAM,
HBA, boot drives, cables, etc. (cables may or may not be included in the CSE-846 chassis). So barely in budget, all used equipment, but with the room you need to grow. I'd also always buy a spare HDD, stress-test it to make sure it's OK, and then set it aside until the day you need it.
Other niceties to explore (as budgets allow and use patterns suggest) are
- Using two SSDs for a mirrored boot pool. The CSE-843Bxxxx series of chassis apparently can have this dual hot-swap 2.5" drive set retrofit into them. Always refer to the Supermicro site once you have an exact chassis SKU from ebay to see what drive kit is compatible.
- Using a 10Gbe network card. FreeNAS has very good network drivers for 10Gbe Chelsio-based cards (see resource guides).
- Two powerful UPSs, one for each power supply, to allow graceful shutdowns if a power failure hits.
- SSDs for L2ARC and a SLOG. However, I'd leave that task aside until you have some actual-use experience and can weigh the benefits and drawbacks of either based on your usage patterns and user feedback.
- Upgrading the power supplies to more efficient models (if the ones you get are basic). Supermicro power supplies can be ludicrously cheap on ebay.
PS: My experience with goharddrive.com has been excellent - i.e. no questions asked if a drive fails a SMART test, a prepaid shipping label is sent to you electronically and a new drive dispatched ASAP. This is a better warranty experience than I received from OEMs. That said, another community member swore off goharddrive.com after receiving several dud HDDs.
PSS: Regardless of how amazing FreeNAS is, how robust the hardware is, etc. never forget to include/implement a solid backup plan. Given your environment, online cloud services may be the best option.
PSSS: Don't forget to fill "empty" HD caddy slots in the CSE-846 with styrofoam blocks that are approximately as wide and as high as a HDD to ensure that every caddy get the same air flow. That's helpful re: cooling for the HDDs! :)