One big problem with a tent in a cold space is when you have to have the door open to work on your plants. All your heat is lost immediately and depending how long you have it open your plants aren't going to like it.
If your light is enough to keep the temps right then you could veg 24/7 but when it comes time to flower you are going to need a heater for the dark period. Any electric heater will work and aren't that dangerous as long as you keep flammables away from it and have your circulation fan going all the time. Some heaters can be hung from the roof but many have safety switches built in and have to be sitting on a hard, level surface to work. Even ones that don't have that feature still have a tip-over switch in them and must be level to work. Oil filled heaters take a while to warm up so the instant on types are better for something like a tent.
Another consideration is how your exhaust fan functions. If it's on all the time you may not be able to keep the tent warm. It needs a heat/RH controller on it so it only comes on when the heat or RH gets above the settings you put it on. Your electric use for the tent will double without one. A speed controller helps too so the fan isn't blasting full on when it runs. It will suck in cold air so fast that the temp will go too low, too fast and the heater will kick in.
You can buy a baseboard heater thermostat and build one to plug your heater in if it doesn't have a thermostat of it's own. Even if it does they don't work so great so having one that you can have some distance from the heater works a lot better. I use all that stuff and it works great to keep my grow room temps nice and even with the lights on or off.
My speed controller is the little box on the left of the heat/RH controller box and I made a similar one for the baseboard thermostat that controls the heater. Pick up the parts at any hardware store. For the speed controller you need a ceiling fan controller and not a light dimmer.
It would help if you can insulate the tent or build a cheap particle board, (OSB), room around the tent so you don't get a blast of cold air into your tent every time the door is open. And don't vent your exhaust into an unheated attic as the moisture will condense up there and melt in the spring. I tried it so I know not to do that.
I'd suppose sparking a 1000 metal halide light and ballast inside of a plastic tent would be hazardous too. I wouldnt store my balled up news papers in front of the blower, but thats just me.
