Everything...
A lot of this will be obvious, for example the most important things is the M42 jumpsuit, then shirt, boots, garrison cap... at that point you start to look like a trooper. After that comes the webbing and weapons. Then what? What is most useful, what will you buy and never use and has no real practical function. Hopefully the list below will help some what. The only modern item listed is a sleeping bag, as much as it seems like a good idea sleeping in wool blankets or period sleeping bags... trust me it's not worth it. You'll get no sleep and to what benefit? The public will have no idea you slept like that, you'll be cold, uncomfortable and miserable! Just hide that sleeping bag during the day. It is your only modern item that you should really bring as part of your kit. Even things like lighters and pens etc should all be period or period looking. There is no reason to have modern items in your pockets or on display.