Belkers another of those monsters few people know about, and fewer even use. They're evil clouds of smoke that kill people by being inhaled, then attacking someone from the inside. Flavorful, yes, but not good PC material.
Firstly, the seven RHD. Elemental-type, and a bad kind of elemental to boot. The class skill list is horrifyingly small (though at least somewhat usable). Seriously though, seven RHD that do almost nothing for you aren't a good starting point.
The other chassis abilities (flight, natural armor, ability scores, size) are nice, but don't match up that well. If you're Large, you are typically going to be a strength-based meleeer, but the Belker only has 14 strength. Similarly, dexterity-based melee typically wants to be medium or smaller.
A belker can turn gaseous for twenty rounds per day: too bad casters got that ability at ECL 5 already. It can also make people inhale it, then attack them from within, but it's a situational ability that deals little damage. You'd be better off just attacking people with your five natural weapons... but there are PC's who do such a thing better.
All things considered, -0 LA works fine.