Probably the engineering tree. Basic engineer you get mines, spitfires (sentry turrets), motion sensors and .. something else I can't remember offhand. Combat engineer gets boomers (remote detonated explosives). They then added two more additional engineering branches which give cloaked spitfires, EMP mines, deployable turrets, cloaking bubbles and a few other goodies. To have them all at the same time would cost about 10 cert points (3 for enf, then 2 for CE, then 5 for both adv eng, or 3 for a single one of them).
To take the higher level skills, you need to have the lower ones already certed.
All medical stuff comes from the medic skill. Basic medic (3 cert points) you can heal people. Adv med, 2 more cert points, you can rez.
Inventory stations would be the AMS, which is a mobile spawn point and inventory station inside a cloaking bubble, for 2 points.