Whats the cheapest protection circuit for this test equipment?
Follow the DAC with a buffer amplifier and as large a resistor as will allow you to meet output spec.
The resistor protects the amplifier.
The "trick" is to sense Vout at the output end of the resistor so the buffer amp can swing as high or low as needed to bring the output to the desired voltage. The resistor max value seems to be be constrained by the need to deliver 3 mA with Vout anywhere in the 0-5V range. 

So eg with 15V available and Vout at 3V,
Rseries max = V/I (15-3)V/3mA = 4K.
Say 3K3.
Now clamp the buffer amp output at about it's supply rails with diodes and the resistor must tolerate P = V^2/R = ((33-15)^2/3K3 = 100 mW!

100V transient output excursions can be tolerated.
(100-15)^2/3k3 ~= 2.2 W.  Size resistor to suit max dissipation and duration. 

Example

R1 + R2 protect DAC.
R1-R2 junction is clamped to supplies by D1 &amp; D2 to prevent overswing from fault output.
Opamp input is clamped to supplies by D3 &amp; D4 to prevent overswing.  

R1 set to max load tolerable by opamp.
R1 + R2 total set to minimum acceptable under fault conditions.  







simulate this circuit &ndash; Schematic created using CircuitLab