Okay...
Desk mat - I have
this one (large size) arriving in a couple of days. The mouse works perfectly without it but the desk itself is a bit of a dust magnet as is.
Leg fixings - depends entirely on your existing legs. Mine were fixed to the original desktop using those steel plugs that you find in pre-built furniture - I had to get them out and then fit them into the underside of the new desktop before I could screw the legs on, but yours may well be simple screws. (Thinking about it, I could actually have just used simple screws with washers, which would have made life a lot easier!)
Pilot holes - yes, I drilled pilot holes first and then used a wireless screwdriver. Screwing by hand would have been challenging.
Cable management - I've already fitted a cable tray, but still need to stick a multiple plug adapter to the underside of the desk with double-sided adhesive before addressing cable management. Once done, though, I'll just have a single power cable visibly going to the desk and everything else will plug straight into the power adapter with the leads sitting inside the cable tray.
Just to add, remember that I'm still recovering from major surgery, and am far from fully healed at the moment. Consequently I'm having to take things nice and slowly, and for some tasks need my son here to help me.