As long as your Lord is unmarked, you can take Cult troops as Elites (remember the Ancient Enemies rule, though).
Rhinos are good to take, but don't expect them to ferry assault troops or to last long if you send them charging at the enemy. Most people tend to use them in a drive-by fashion now, driving the Rhino toward the enemy lines (using Cover where possible) and then disembarking a shooting unit in later turns. Whether the Marines have CCWs or Bolters is up to you, being able to plant the full squad + 2 Meltas or 2 Flamers or 2 Plasma Guns by the enemy is plenty lethal.
Since it's your only vehicle unit, I would suggest getting another Rhino or not using them at all. If the enemy list is full of Lascannons that one vehicle is the only prime target, you'll have to keep it out of sight or expect it to 'splode early in the game.
I think your purchasing plans are fine. I'm not sure what you're asking about the Havok squad, though. Any Marine can be used in a Havok squad, there are no specific models for them (other than the ones featured in the Havok Squad box, but those can be used as Havoks, Marines, Chosen, et cetera, so long as the weapon limits are followed).
