Sentry Guns is in the Special items shop. Just simply press 4-2-5. The Sentry Gun costs 1300 points so yea. If you choose the Heavy Gunner class, there's an item called "Sentry Repair" which is only 500 points. The Sentry Repair repairs your Sentry Gun.
RC-XD is 4-2-6 if you didn't know. I understand it may be hard to know where is what item on the shop but sooner or later you'll get used to using the Human and zombie shops. No problem. The RC-XD costs 600 points so yea.
Care Packages would be great although there are positives and negatives. The positives of implementing the Care Package on the mod is, you will get something valuable in the care package that you'll be able to use against the zombies. As for the negatives, care packages can kill yourself and enemies so watch out where you throw it, teammates might steal your care package when you aren't noticing or are trying to be funny, you might get something useless in the care package, and the enemy team (which are the zombies) might be able to steal your care packages. In my opinion, I'm not really into adding Care Packages to the mod as I've already stated reasons.
Valkyrie Rockets - I don't really see no point in using it. I mean there are explosive weapons and C4 on the mod that you can use instead.