Evolve?

Attack 15 is the tits! I think it's a good go one. Good luck!

I don't think CP is a meaningful measure of the effectiveness of a blissey as a defender. All you care about is def and sta. The big pink pillow isn't going to KO anything, it's all about slowing down the attacker so they time out, or just decide to go for some other gym that has something less time-consuming to beat up.

I would definitely go for it. That is a very good Chansey, and I think Blissey may be even better than we thought, so no reason to wait. Maybe consider keeping the 30 candies (and not powering up) to see if you can find another Chansey.

The defense stat is glaring? Think about what it means for a second: 9 out of 15 doesn't mean that it has only 60% of the defense of a perfect specimen; it means that it has a defense stat of 185 out of a perfect 191. In other words, your Chansey is 97% of perfect on defense (actual stat, not IV), and 100% perfect on attack and stamina.

Maybe where you live, perfect Chanseys are a dime a dozen, but where I live any Chansey is hard to come by, even during the V-day event. The question to ask yourself is how long will it take to find/hatch a better one than the one you have?