Best way to hang a server rack in the attic
Last place I'd put one, as a network/wiring pro. Basements (off the floor/above flood levels) are good, attics not so much, and anywhere you plan to inhabit is usually a poor choice (the noise gets very tedious very quickly.) Thus, an attic that you think will be cool enough because you'll convert it to a room that you will use is still not a good choice.
Apart from the background temperature of the space on whatever a hot day is in your mild climate, have you considered how you will deal with the heat this will be pumping out continuously when you've loaded it up with 70 KG of stuff that uses power?
As for mounting it, either drill into the masonry wall and use appropriate masonry anchors and bolts, or put planks across the framing it's sitting next to and use lag bolts. If planks on the front of the framing would upset your finishing plans, mount a plank (roughly 37mm or 1.5 inch thick) between framing members to be behind the finish layer you plan, and get that finish layer (or a piece of plywood the same thickness as your planned finish layer) in place behind the rack, before mounting it.