You should be able to expand anything you like with a few exceptions.

You can:

1. Add SATA PCIe cards -- assuming you have open slots (the Desktop should have at least 2 SATA connections on the motherboard already)
2. Add additional hard drives (although you may need internal adapters depending on the number you want to add)

It will be challenging to:

1. Upgrade the power supply (I think the last Optiplex with a standard 24 pin connector was the 7010.
2. Upgrade the case (Motherboard is most likely not standard ATX and the front connectors for the power switch, etc. aren't standard).
3. Upgrade the motherboard. (Likely won't fit a standard ATX motherboard without case modification).

That being said, I ran a i5-3470 in an Optiplex 7010 for 4+ years with 2x SSD's and 5 HDD's...even swapped out the case. I called it my "FrankenDell" as the power switch looked like a periscope sticking out the top of the case.

Yes, FreeNAS-11.2-U5, was also working on U4.

I literally just plugged it in and plugged a drive in to it and it worked. I also have switched drives between the controller and the motherboard and that works too without breaking my pools. Support for the Marvell 9215 chipset was apparently added in FreeBSD 9.2... Not sure why you would be having trouble, does the card work in a different PC?

Oh fuck my bad, I Just double checked and THIS is the one I got... Slightly different but super close image wise. The one I got has the Marvel 9215, the one OP linked has Marvel 9235.. But based on a quick google it looks like the 9235 was added to be supported in FreeBSD 9 as well...