I dont know how your setup will be but I'm assuming you'll be in an untreated space (acoustically speaking), I wouldnt be afraid to forego monitors and get some top end headphones. I have some AKG 702s that work well for mixing, you dont have to worry about your room hurting your mix.
Upgrade...
Pretty good. Drums could certainly come up, would give the whole thing more energy. And from memory, particularly on Koloss the guitar is even dirtier than that. Maybe try something like a voxengo varisaturator vst at the end of your chain. It should give you some dirt but not get in the way of...
Hey guys and gals, here's a mix I bled over for my band. Done on a complete budget.
HERE it is: https://soundcloud.com/defytheocean/gold-green
Drums: superior drummer 2 (custom kit mix of NY Avatar & metal foundry & Drums from hell)
Guitars: 6505 ->Marshall Lead 1936 -> SM57 & Heil PR30b...