open the console and enter this:

ent_fire player addoutput "max_health 999"

yes, you can change that number to whatever you want. lower it to 50 if you want, or increase it to 9999 (the hud will not display numbers in the thousands on your hand, it will show it in vehicles).

as for increasing your maximum ammo capacity, find the skill.cfg file in the local game files
steamapps/common/half life 2 vr/hl2/cfg
scroll down to the weapons section and look for something like this:

sk_plr_dmg_ar2 "8"
sk_npc_dmg_ar2 "3"
sk_max_ar2 "999"
sk_max_ar2_altfire "99"

as you can see here, I have already edited my file. sk_max_ar2 is the maximum ammo capacity for the combine pulse rifle, and altfire is the capacity for the energy grenades. I think the default used to be 60 reserve for standard fire and 3 for the grenades.

now change the values of the other weapons to whatever you want. lower them (why?) or raise them. you can even edit the damage values for specific weapons, make them do even more damage, make them do less, make yourself take less damage from them, or more, your choice.

I hate wasting food, and medkits are like food. ammo too, its gun food. now, if you like exploring the game world and scooping up every drop to carry with you forever, you can. it's especially helpful for playing on hard going up against striders, gunships, or the prison turret defense section. also especially helpful for the leadup to ravenholm, where I still ran out of ammo. the magnum finally carries more than 12 in reserve.