2 claws with Imp Grab sounds good. Want to add an automatic bite when grappling?
A crab spider uses its legs to wrestle prey, not to damage it, so I don't think a claws attack fits very well. Instead, I'd suggest the same approach as the SRD squid's arms (0) and bite (1d6+1), except with "legs" with improved grab instead of "arms".
A crab spider's forelegs are very powerful, and they can wrassle opponents several times their weight, which suggests either an unusual high Strength or a racial bonus to grapple. I'd prefer a racial bonus.
I'm OK with either an auto-hit bite or a bite with a circumstance bonus to attack when grapple.
Dex-damage poison to paralyse its prey is a fine idea. Makes a change from the other spiders.
EDIT: Although I would mention that crab spider venom is highly potent, so it often kills within seconds (I'm guessing this is because crab spiders are very slow and don't have webs, so usually only get one shot at their victim as they can't chase fleeing prey or entangle them, so must kill or immobilize them on the spot.)
EDITED EDIT: Maybe Dex and Con damage?