Wemotec Minifan PRO and ARC2858 motor on 6 cells will produce a sustained 2.5kg thrust which is in your power range requirement, the amp draw is manageable too at 85-90 amps with a full 54mm exhaust nozzle,as you are looking at VTOL you wont need to constrict the exhaust down much which will lower your amp draw to around 80A. Minifan comes with clip on intake lip so you are pretty much covered for 100% efficiency in the setup.

The wemotec rotor is pretty much the most efficient 70mm out there, the fans are lightweight and very reaonably priced, with ARC motors and heatsinks you will have an affordable lightweight setup.