Before you do powder coating, remember to drill all holes on the case a little bigger, as the powder fills them up and can make screws unable to pass through them afterwards. You cannot powder coat a case yourself, so you're going to have to contact someone who can. Usually people who make steel tables and chairs, as well as bigger car manufacturers will be able to assist you in this. On the other side, if you can buy a case with complete black insides, I'd suggest it far over modifying a new one. It's going to be expensive to powdercoat a new case, and there is a large chance of the case being pulled a little skew. Powder coating gets baked onto the surface at high temps, and as such can cause problems.
Overall, powdercoating is an extremely good coating, but the troubles of getting through it is too much. I'd not recommend it.