Let's see:
Kavs: Hopefully digging will be the 'main' job of a miner/builder, since the scientists and engineers need the resources to research and build things. Also, mining can be used for funny ambushes-from-below. Besides, I'm planning of adding 'artifacts' that are objects found underground, that can be researched separately to gain special stuff. If you've ever played Space Station 13, you'll know what I mean. The building aspect will probably mostly be about fixing broken walls and building reinforcements, unless the builder is exceptionally good at avoiding death and starts expanding the base.
As for the buttons, I'd really like to use the context idea Dragonmaw mentioned, but I'll have to see to what extent the players need to be able to interact with the environment. Currently the biggest problem is stuff like bombs, jetpacks and cloaking devices - you'd need to be able to interact with them while still being able to use other items, thus making the 'context' button unusable here.

This is how the HUD looks currently. If I use my original idea, it'll have four icons instead of three, but that is to be seen. (If you're wondering why the gun is there, every class will have a weapon of some sort, it's just that soldiers/fighters have much more powerful ones.)(Also that's a syringe, not a sword, darnit!)
Here's a silly explanation your for the icons:

- The first icon is the 'special ability' of each class, so the icon will change depending on the class. The Miner/Builder has the 'build' ability, the Scientist has the 'heal' ability, and the Engineer has the 'fix' ability. I'm not sure what kind of an activateable special ability would fit the soldier, all suggestions are welcomed!

- The second icon is, unsurprisingly, for attacking. As I've mentioned, the weapons vary a bit depending on the class - the soldier has a more powerful gun and the miner can use his gun for mining.

- The third one is currently WIP and pretty ugly, but the idea is that this is for the 'context' command, kind of. Basically when you have this icon selected, you'll interact with items. I'll have to think of the arrangement of the icons a bit, since this should probably be the first!

- I quickly drew an ugly backpack do demonstrate the possible fourth icon. Basically this ability would let you to pick up, carry and throw things. It'd be used for larger carriable things like artifacts and bombs, but also wearable bonus items like jetpacks. I'm not entirely sure how this'll work out, we'll see.
Thanks for the feedback, I'll make a post about today's updates later!