Question about Level 1.

Will my items in the stashes remain? I am asking because I have a load of stashes with loads of items, it's absolutely insane how many items i have in the arena stash alone, basically every single item I've collected until now except for those where I made special stashes for since I was too far away from Rostok.

I just want to know if everything in all my stashes will remain.

Yes, everything in your inventory and stashes remains between levels.