Hmm, lets see.
1) Can't help you here, as I haven't seen the new codex yet. I imagine they have an armory.
2) Power weapons are basicaly a close combat weapon that ignores non-vehicle armor. So, if a Sargeant were to attack a Space Marine with a Power Weapon, the Marine would be unable to make an Armor Save against any hits from the Power Weapon that cause a wound. Invulnerable saves may still be taken.
3) Sponson weapons, as far as I know, are bought in pairs. So, a Leman Russ Battletank could have a pair of Heavy Bolters (one on each side) OR a pair of Lascannons (one on each side) OR no sponsons at all.
Hope that helps.