There is currently only 1 mag you can use with the KJ. There are 2 more mags planned to be coming out soon, but we do not know how well they will preform in terms of gas use compared to the awesome mags we already have.
You will want to get a tight bore inner and new hopup rubber. If your going to be gaming it, you might need an NPAS to lower the FPS to field standards. There are a couple other things you might want but are not really needed right away. If you get a hold of Amos on here, he can sell you the rifle, an AI kit, and all the upgrades you will need. AND he will install them and test before he ships to you.
You WILL need to keep some silicone grease and get an AI propane adapter. Grease will be for lubing the mags and the BCG. Both of these can be obtained from AI. www.airsoft-innovations.com
btw, I am willing to bet my left nut on this