They are there as a sort of mobile reserve, meant to be able to handle almost anything they need to
I've used a Landraider in this way before and it can be very effective. It allows you to use the lascannons for a few turns and then deliver a deadly payload into any fight where it is needed most. Great for breaking through enemy lines and smashing late game objectives. I use Berserkers though, nobody is better on the charge and more worthy of a LR.
As for the troops, I've never been a fan of taking any CSM squad at less than 10 models. The extra bodies makes a big difference when in combat and rapid firing and of course the extra wounds help, not to mention the whole special weapon situation. In this instance I might recommend Plague Marines for the job. They are a unit you can get with min sizing and not have to lose a special weapon as well as tough as nails. You could get 3x 7 man PM squads with double melta and fist champ in a rhino with a few points to spare (32?).
I get not wanting to drop below 4 scoring units, but if you were to drop the termies for an 8 man zerker squad with fist champ like I mentioned you would have 4 scoring units. Of course the LR would become heavy support, meaning you'd have to drop the single oblit, but the LR can fire 2 las for a few turns just the same. That gives you some extra points to spread around as well. Combi-flamer PM champs? yes please!
Sorry if that seemed like me re-writing your list, not my intention. I'm just throwing some ideas at you about different units doing similar jobs.