The first question is a tricky one in so many aspects. It all really depends on the
Hero and
item scaling in the map. If you design the waves first, then you can make the Heroes and items scale accordingly to the difficulty of the waves.
Remember to have scaling damage on skills of the Heroes. Test your map a lot, it really helps and gives an idea on how to fix balance issues.
Also need to count how much gold or any resource the creeps that you kill give. And then you need to balance the items based on how much gold you gain in the map.
Usually you see item scaling normal items/magical items/rare items/epic items, etc.
Oh and if you want to make more like a custom scaling system, you can change stat gains in gameplay constants.
The biggest issue I have seen in the past or made myself is that with the
normal armor damage reduction multiplier unit's become too hard to kill. You can lower this from gameplay constants though.
So I would personally try to make a custom damage system using a DDS.
Damage Engine 5.5.0.0
And for the waves, I like to have Boss waves and waves that contain Champion/Lord/Epic/Unique units, with personalized abilities: Higher damage, Aura, High health, etc.
Then for some ability balancing. Area of effect spells that deal damage, should always deal less damage than single target spells. It's a good ground rule, you don't want to have useless Heroes in your map.
If you want to make passive abilities that proc when the Hero "takes damage" the percentage should be set so super low. Otherwise the passive will proc all the time with only 3-4 creeps hitting the Hero.
Making passive stun abilities (i.e bash) should also be made with super low percentage and probably should think if it should be able to proc on Bosses.
You could also design the Hero-types on your map: "Fighter, Tank, Off-Tank, Healer, Support, Mage" Based on the designs then make some basic stat lines and "set of skills".
For example: "Mages are great at dealing damage, but die easily".
Anyways I personally think Hero Siege/Hero defense maps are pretty hard to balance. Hope this helps and gives you some ideas. Good luck on your map making process!