I'm looking at spending a few days there next January as a precursor to going on to Argentina and sailing back to Southampton so I'd be in in what you decide.
From what I can tell you need to spend a few days at altitude to acclimatise before doing anything too strenuous, maybe in Cusco. Book your Machu Pichu slot whilst you can.
I've found this company but I don't know if they're any good. I haven't spoken to them yet.
www.llamatravel.com