Koi pond maintenance is not too hard but I've found a few items that come in handy.
A salt meter; these are very useful if you want to handle any koi health issues your self. Salt will kill most parasites, but sometimes you need to up the dose for the out of control parasites, you can tell if you have too much salt in your pond because the plants will burn. This will not mean that you are hurting the koi however. Remove the plants if you need to dose at a high rate. A strong dose of salt is a meter reading of 0.3 but for really sick koi a short time in 0.6 is very effective. A high dose should be reach gradually, we are trying to safe the koi not shock them. You can dip them in a salt bath of 0.6 for about 10 minutes, and then put them back in the pond or you can bring the whole pond up to that level. A 0.3 is very effective and should be a good place to start.
A ph tester or a ph test kit; is good to have around you should know what your ph level is that comes out of your water supply (hose or well). If you fill a new pond up and find that your ph is above 7 don't worry is will disapate in about 24 hrs. It still may be higher but most koi are perfectly happy in a pond that is a little high and do get used to it after a while.
An Ammonia test kit or meter; also good to have on hand. This will allert you to ammonia spikes and keep your koi from suffering the effects of ammonia burn.