Some gear I have found a must have on my K-3 II.
1. Quick detach strap. I use Peak Design. I hate fighting the strap when using a tripod.
2. A good tripod and head combo.
3. Extra batteries.
4. Grip (Not a must have but gives you AA battery option and if you shoot a lot of portraits it gives you vertical controls.)
5. SD cards. I use SanDisk and PNY. Most of mine are 32 GB. I do have a couple of 64 GB.
6. Carrying options. This is highly personal. Due to shoulder and back problems, I can no longer carry heavy shoulder bags or backpacks. I used to love my Domke bags but they just don't work for me anymore. I now have systems I can pull behind or wear on my waist. Highly recommended Think Tank Photo products.
7. Remote trigger for tripod use.
8. Flash. I have the 360 II and it is an awesome little unit. It is small but plenty powerfull and recycles faster than the 560 II. I shoot a lot of weddings and it has done a great job. I use good NiMH batteries in it. Panasonic Eneloop are by far the best.
---------- Post added 10-09-16 at 06:13 AM ----------
Having read your planned lens needs I would recommend the following bag options
Think Tank Speed Demon. It can be used as a shoulder or waist pack. It easily carries a body and a few small lenses with a flash. I am able to get my K-3 II with 18-55 attached, 15 and 70 limiteds, 360 II flash and my DA 50 1.8 in it.
We also have several of the Think Tank modular components. The Digital holster 20 would be a great option for just your body and the 18-135 attached. It does expand on the bottom if you would like to toss in a small prime.
Last edited by sarge; 10-09-2016 at 06:15 AM.