Do your items ever get destroyed in combat?
Do you get equipment back from dead soldiers?
Do items carried by squad members (e.g. armour, weapons, medikits) ever get destroyed in combat?
I'm wondering if I need to worry about having funds available for replacements, or if I can assume that I'll only ever need a maximum of 6 (annoyances in transferring items between units notwithstanding).
No, they do not get destroyed ever. As Daedalus' link points out, if you enable Second Wave mode (after beating the game once) you can toggle an option so that squad members' equipment is not recovered on death.