Never in my wildest dreams did I think I would ask this question...
A
Another thought, there are lots of other basics these guys need right now that most people are probably overlooking. Eye and ear protection are important, so are good boots and gloves. Also socks, send lots of socks.
You can get further, with coverings for arms, neck, and groin though they are not quite as typically used.
As for helmets, more or less whatever your host nation uses for their military.
Isn’t it better that you give money to the Ukrainian military though, so they can get what they need? How will you even get the equipment into Ukraine now?
Recommend velocity systems scarab plate carrier, and something like hesco 3810 multi curve plates, back and front.
I wouldn't recommend side plates and groin as they greatly reduce movement, but if you're gonna stand around and get shot at, then maybe?
Opscore ballistic helmet if money is no object?