It's easy to know whether a colonics tech is good or not - just visit them and see if you like them! colonics are so easy to adminster, very few tricks to it, that anyone can be proficient so long as they care enough to be good at it! and that you can tell by how comfortable you feel with them. If in doubt, just leave and find someone else.
Oxypowder is not great for impaired kidney people. You'd have to take very small doses and that rather spoils the effect as an intestinal cleanser. Stick to fiber and herbs for the most part. colonics and enemas are fine.
To give the kidneys a really good break, your diet temporarily should avoid spinach, rhubarb, beets, asparagus, all beans except for mung, and all protein but for broths, eggs and fish, and the rest you already know about. :)