Item Level Break Points?

So for the first time in my Path of Exile career I'm actually mapping in hardcore.
In order to upgrade my gear I only pick up currency, perfect roll white jewelry and potential 4-link whites with the best possible base item to alch them.

When I got 5 crappy rolls on the same item type, I vendor them to reroll them.
If I understand it correctly item level influences the range and type of affixes/suffixes an item can roll, and because the 5x magic/rare item vendor recipe always creates an item with the lowest item level of its ingredients, here's my question:

What are the item levels that I should watch out for that generally allow for more powerful rolls?
Can I mix a 59 item with a 70 item or can the 70 item be already significantly more powerful?

There's so many different suffixes and affixes in this game, that I don't want to check every single one of them and wondered if anyone of you could tell me from experience which are the breaking points when gear starts getting a lot better than the previous tier.

Thanks in advance

-Meow

For me it is 72 for jewelry and armor, 77 for phys weapons and 79 for magic.

http://pathofexile.gamepedia.com/Item_Affix

You'll see the affices and what level is required to get them. Up to you to decide your "breakpoint".