When is using a suspension fork worth it?
Proviso - my advise presumes you are not looking at forking out $2K or more for a bike, and probably significantly less. At a high price point I might suggest suspension. I also presume the gravel section is well maintained with average (pea - grape) size gravel (Where I ride, we sometime use logging roads, the "gravel" is stones about 2"-3" across.), and straight rather than technical single track

Advantages of suspension - Softens the ride on the arms making the ride more comfortable, holds front wheel on the ground providing better handling, especially  while cornering. Allows you to ride a straighter line than you might otherwise be able to. 

Disadvantages: Heavy and very expensive for good shocks, need maintenance and cheap ones don't really work well. Absorb energy while riding slowing you down - can be mitigated with a lockout, but even ones with lockouts move a bit. 

You only have 200m of rough ground to cover, so a suspension fork is probably overkill. If I was buying a bike for that commute, for the same $$$ you will get a significantly better bike with no shocks, any loss of time over 200m of rough ground would be made up on the rest of the 5 miles because of the better quality bike. Cheap shocks won't really help with the comfort, so unless you can get to at least a mid range in price, don't bother.

I would consider padded hand grips and gloves - cheap and easy to "tweak" if needed. Also big tires will help with the rough ground and gravel - I would look at 1.75"-2" slicks, and probably a 29er MTB of cyclocross style wheels (depending on your height).  

Consider a quality steel frame if you cannot afford carbon - they absorb vibrations much better than aluminum. The problem is many shops do not stock, and know nothing about steel and will tell you "stories".