upgrading a CM16...I don't know..seems like a cheap gun that it's just a starter..
maybe a spring..and barrel..
vest, easy. Don't get black if you are planning on wear anything camo in the future. I made that mistake...and usually people like plate carriers.
Mid caps, yep, you need around 3-6 for a drop in game. Just something cheap and feeds is good.
eye protection, get something that WILL let you ads easier. mesh mask did not work for me to ads...some soft cheek full face masked seems to work good like the flex 8. But this depends on if you are playing CQB, if you are, it doesn't matter. just anything rated and anti fog
gloves, anything that fits good and grips good. it really just serves as a hand protection..
Boots, get one with ankle support that's really about it.
headgear is a maybe...i just wear a hat..but again, I don't wear a camo uniform anyway since I only have time to go for drop in games for a couple of hours..
knee pad..damn it hurt without one.