Well, first off I would probably suggest getting a rifle that is chambered in .223 Wylde. Then you can safely shoot both civilian .223 and military type NATO 5.56mm. People will tell you that they are all interchangable, but they aren't, and it's been discussed, ad nausem, on the forum here. I believe most of us are on the same page that the Wylde chamber is the recognized "safe" way to handle the pressure of both rounds not have to worry.
As for ammo - all you can get your hands on.
Seriously, I have three .223's and I currently have about 1200 rounds at the house. I will probably add some more before the election, just to be on the safe side. I have heard plenty of people plan on 500 rounds for each AR/AK style rifle, and that seems like a good round figure to me.
As for parts - well, that opens a can of worms. How much is enough, and how much is too much? Opinions vary. Personally, I have started ordering a little extra "replacement" stuff for a tacklebox each time I do an online order. My goal is to have enough "high use" stuff to build a complete new model, if I had to, from closet stock. I don't think you are planning on invading a country anytime soon, so you can probably add some of this stuff over time. You certainly don't need to rush right out and purchase every available thing that can be replaced right up front. Buy a piece here and there, over time, as you use the weapon and get more familiar with it.
Magazines - Always good to have plenty of those on hand. Most guys on here probably have 20 or more lying around the house... LOL I have about 30 or 35 between my three weapons. Probably should order up some more thinking about it....
Extra Magazine springs - If you use your mags alot during training / practice, it's a good idea to get some mag springs to change yours out every so often. I am sure there is an acceptable time / round count for the change out, but I am too lazy to look it up and I don't want to misquote it. Someone here will know it for sure....
Springs and pins are cheap. You can get a spring kit and a pin & detent kit from places like Brownells and MidwayUSA for probably $20 to $30 total. It would be good to have on hand in case you lose/break/foul up one while doing whatever with it. Generally speaking, if you are putting through several thousand rounds a year in training and practice, you should plan on changing them once in awhile anyways.
Replacement BCG - Bolt Carrier Group. It can't go bang if this is fouled out. They aren't cheap, and if you maintain yours, it will probably last a long, long time. But, if you want to be super prepared, having a back up wouldn't hurt.
However, you can buy a complete BCG refurbish kit for much cheaper than you can buy a spare bolt and carrier. Having the kit would require a bit of time to do the work, where having a replacement would be a drop in fix. Not super critical, but it wouldn't hurt to have one somewhere down the road.
Pipe cleaners - good for cleaning the gas tube. Probably the most overlooked part of the system that very few people actually do the work on, but it's actually the backbone of the weapon system.
Extra brass brushes and cleaning products for maintenance. I would stay away from the famous WD-40 and magic wonders like that. It creeps and "can
" ( and watch this start some controversy
) seep into one of the top rounds in your weapon, should you keep it loaded, and keep the powder from going off.
I am sure I am forgetting something - but someone here will chip in. The weapon system itself is pretty easy and pretty reliable. If you take care of it, it will take care of you, just like most everything else.