hehe, checked it, played 59 hours since the release
But meh, you didn´t answer my question, where do I find the prices and detailed information about the things
Yes you must buy C4 like grenades and is more expensive than grenades, C4 also seems very reliant on certain situations. If you are defending against alot of enemies it'll prove useful to place, hide and watch, then detonate as a group of enemies step near it, also if you're good at running and gunning you'll probably find it's decent for placing on enemy veichles.
But I enjoy using Proximity mines and Anti-tank mines , you can drop them and forget about them, maybe not as devastating but highly effective and doesn't need any of your attention once placed, so if medics only option is C4 I'd say start certing a veichle. I myself am certing both Engineer and the Scythe. I think C4 most likely takes up your utlity slot too, never played Medic so I don't know what you'll have to replace to use the C4.