Well milsim wise your going to need stuff based on what sort of event it is there is some cross over but general stuff you will need.
Plate carrier or vest, some means to carry mags. You'll also need some mid caps since most milsim events require them. BDU's I'd pick up some woodland on the general idea that it's a cheap commonly available camo. But allot of milsim require specific camo. So for instance I just bought tri color desert BDU's because of the upcoming OP Irene in October requires my team to wear them. I don't need them for anything else really, and they won't blend well here. But I needed them to match team so I bought them. That's why I wouldn't get too tied to getting BDU's right now, buy something that will work for most events, then buy what you need to for specific events. Radio is a good idea, I just ordered a Baofeng UV-5R from recommendations all over, though smaller games I've found often others don't run radio's. Milsims I expect most do, but it may vary on who is organizing and how in depth it is. Helmet is a good idea in general, though that said in woodland environment you may find it less useful than a boonie hat. Generally a helmet is great in CQB and the like, but it depends on your play style how prone to whacking your head you are, but I wouldn't discourage a helmet but a boonie is lighter, less hot etc. Goggles for sure, up to you but I've recently become a fan of goggles with a built in fan, they aren't very common but after many years of fogging issues they have solved that problem for me. So I'd give them a nod, whatever you use make sure it's full seal and ANSI certified. Mask wise another I'd highly recommend, in my opinion only second to goggles in equipment. I like the mesh masks I buy them then modify the shape so they actually fit my face comfortably. I can't tell you how many times I see guys complain they aren't wearing them because they aren't comfortable. Then I ask them if they adjusted it, and only get a puzzled look. It's metal mesh and quite pliable, bend it to fit your shape of your face and head and it will be tons more comfortable than stock. I'm not sure what they use for a mold but it certainly isn't shaped like my head;)
On Irene that's the op I'm prepping for, but beware when registration closes you can't get in. So if your goal is Irene I'd get your registration paid sooner than later to make sure you get a slot. I know it's a good chunk of change and it's early but I suspect registration will close pretty soon from what other guys are telling me.
Take care!
Luke
Oh and on Irene depending on the team you need pick camo based on that;)
