Stuck with Z axis printing directions

Hello there guys!
So, ive built my first 3d printer! (wo-hoo) But cant get my head around the way Z axis should be configured(booo)..
As you can see from build, top left back corner is my (0,0,0) point.. well thats how i want it to see.
XY axis move fine, but Z is acting up.
When I change config file it either moves down the height of the model, then "printing" up.. or goes all the way down.. or somth wrong with endstops.. im confused.
Please help! Im using Marlin and heres config.

Wow, a wooden CoreXY!

Does the z axis move in the correct direction when you do manual moves (i.e. the bed moves down when you move in the +Z direction)?

Wow. I am impressed with this build... very rustic. Can you build this with just basic hand tools and a drill????

Where is your Z endstop physically located? top or bottom? Are you using a bed sensor instead of an z endstop? Knowing this will be necessary for someone to help you.....