I got a really good deal on a refurbished ThinkCenter M83 SFF from Lenovo Outlet a while back (cheaper than buying the equivalent components myself). I decided to significantly upgrade this computer and use it as my primary workstation.
I added a graphics card, wi-fi card, a ton of storage, maxed out the RAM, and upgraded the case fan and cooler. I may upgrade the TFX power supply in the future, but the current one seems to be working fine.
Also, this chassis supports a regular ATX motherboard, so I can always re-use the chassis in the future for another build.
Didn't really have any issues with the build, although the case makes cable management really terrible.
A few notes on part choices:
Quadro K1200; there aren't that many low-power half height single-slot graphics cards out there right now, and this is one of them. It's not the most powerful thing in the world, but it handles 4 1080p monitors and some light gaming just fine.
Oyen Digital JBOD array: Because it's a small chassis, I couldn't quite fit all of the HDDs I wanted in the case. While this has an internal RAID card, I'm just using as a JBOD array.
StarTech PEX2M2 PCIe 2x M.2 adapter; This is a really nice piece of hardware to add more solid state storage. Would highly recommend it.