how much natural magic resistance does a hero have? (is there like a magic dmg penalty towards heroes? tinkers laser and any pure dmg spell are the only spells i know that do the dmg listed)
do you think quelling blade is worth it? (i don't, used to playing without one. tell me why if yes)
whats your favorite item setup at the start regardless of what hero youve chosen/been given? (i like courier/ror/iron in most cases)

All heroes have 25% magic resistance

^ This is the magic resistance towards heroes

Tinker's laser, PA's dagger are some examples of pure damage.

Quelling blade is worth it on carry, high damage heroes.

If you were to get ganked, most melee usually cut trees to run into. This also gives them the advantage of not needing full vision to land a hit unlike range which needs to cast attack animation (Longer)

Depends on heroes
Carry - you shouldn't really be going courier etc. Some carry requires circs and some just require tangos/flask. probably my choice of items.
Quelling/tangos/flask for me

Support - Chick/tango or chick/ward depending on hero if it has heal or not

25%.

Quelling blade is worth it on any AFK melee farm hero. (or melee who can swap to range)

As for starting items, i rarely do support. My role is usually ganking.
Starting usually 3 branches, tangos, salve and 2 clarities.