Thoughts on a cellar grow please.

I'm in the lucky position of having a cellar at my house and I'm planning on building a grow room H 160cm (5ft 3) x L 120cm (4ft) x 120cm (4ft).The actual cellar itself is about 2.5 times that size so the grow room will be self contained within it. I'm planning on having an exhaust pipe into the cellar with an inlet from outside and a second which I will draw air from inside my house for the winter as I'm growing in the UK. I will be running a 600 HPS and I wondered what the likely temperatures in the grow room will be in the space (5ftx4ftx4ft) from running just the light alone ? The actual cellar temperature is usually very cool, 5 C -14 C so I'm hoping the light will heat the grow room to a suitable temp when on but I think I will need some sort of heater to keep the temp up during lights off. Any recommendations as to how I use a thermostatic heater with temperature controlled fan unit?Is there something that will vent and heat in one go?

I'd use one of those oil filled heaters to maintain the temp. And I don't really want to guess the room temp with the light on. Best way to find out is to hang the light and turn it on.