Which Optiplex generation? Dell uses non-standard front panel connections, though at least in the Optiplex x010 and newer they're down to one board for audio / USB / activity LEDs, but the only way the motherboard indicates errors is with their proprietary power button / multi-color LED board.
The Dell Optiplex x020 (and presumably newer generations) are 12V-only (sort of- there's another power supply connector w/ 5V for the SATA ports, PS on/off, and power good signals) motherboards. The stock power supply is a rather wimpy 290W.
These are the 7010 motherboards. And all their "proprietary" crap is easy to fix. In the role they are being used (low power server duties), a simple jumper on the front panel connector takes care of the error, and no front panel devices are actually used.
The propreitary power cable is easy to fix as well. One more jumper.
The stupid 5 pin fan connectors are easy to fix as well.
On the plus side, these are sparse motherboards with two pci-e slots (x16 and x4), 4 RAM slots, Intel 82579 LAN, and take a standard ATX PSU.
Oh and they can be had for <$10 a pop if you look carefully or buy a few together. With IB i3 and i5s so cheap (I bought the 3220s for $10 each), they make a pretty decent machine. The i3-3220 is 3.3.GHz 2C/4T and can be given up to 16GB on these boards.
A true server type platform where I can dedicate that kind of horsepower to each VM, is gonna cost more than running multiples of these physically. The total power consumption of let's say 5 of these, is slightly higher than 5 equivalent VMs, but chances are, the performance is higher as well.