I've done a number of water loops for customers but never one for myself. And while I wouldn't consider myself a pro at it or anything I will try and give some opinions.
Each water setup always runs into its own set of problems and you actually need to measure and order parts out and then hope it all fits together in the end. When it doesn't, you gotta order more parts and wait more. lol
From the looks of it, I would cut two holes at the top and mount the radiator push/pull style to the top with the two tubes going out. Since you already mentioned you wanted a 120 rad in the front.You can stuff the HDD into the 5.25" bays and cut out a segment to let the wires through from the left side of the case if you need to.
Have you considered cutting out the 5.25" bay and then mounting the radiator there instead and still having a fan for the HDD cage and leaving the HDD cage at the bottom there?
I would also put the reservoir right at the bottom next to the HDD cage right now and try to loop it pumping up and then through the radiators and coming back down into it. Putting it outside would probably complicate things a lot more. Either next to the HDD cage if you leave them, or next to the radiator there if you cut the HDD cage out.
Is this a steel case? After working with steel and aluminum frames for custom water loops, I will say 100% that I would never want to do work with a steel case again if I need to modify anything. Aluminum is just so much nicer and easier to work with.
If you're drilling stuff on the steel case, I will recommend that you actually spend some money and get cobalt drills. If not, depending on the case, you might be in for a bad time.
In the chronicles of my life there is a legend in which I change 'history' into 'herstory'.