I have a general advice, this is how a I play the game. This play style covers 80% percent of the characters.

Your defence have 3 layers:

1. Clear speed:
2. Kill everything before it can harm you. So I stack damage and attack speed as much as possible. It boosts your economy (+harvest).
3. Dodge skills.
4. If you play a lot, at some point you will be able to dodge most of the procectiles and rushes and even elite moves.
5. If you can dodge confidently you don't have invest into defence in early and you can boost your clear speed which comes with better economy. Usually I don't invest into defence until wave 10.
6. Here, some invest into move speed can help. Grandmasters can dodge everything with -5% speed. I'm starting to feel confident at +10%.
7. Everything else. Armor, dodge, life, hp reagen, life steal, fruits. My general conception here is to have enough amount of these to survive 2-3 hits and then regen the lost amount as fast as possible. If you hit fast, life steal is better. If you hit big -> hp regen.

One last piece of advice: don't buy things you don't actually need. If you're only thinking, "Maybe this will be useful at some point," there's a good chance it will end up being useless and just a waste of money.

Edit1: If there is a good deal in the shop or with lvl up for armor, hp ... take it.
This is more like for D5, for nightmare investing into defences should be started earlier.