Late on this, but think the info is helpful to others:

Typically most kits are 35w. The newer kits offer 50w (which IMO, is overkill).

You'll need 35w bulbs for 35w ballasts. I wouldn't mix and match those.

With bulbs, usually it's best to replace both if one go out. 99% of the time, the colors won't match if you change just one.

At the prices that kits/bulbs are going for now, it's more affordable to just replace an entire kit!