In my honest opinion, I don't think going from a centrally heated house to a garage is a good idea. Garages drop below 0 just like a shed would, and they have cold concrete floors and many get damp.
If you could heat the garage sufficiently and keep them off the concrete floor it would be okay, but garages are usually draughty and uninsulated and lose a lot of heat. You can't have the radiator too close to the bunnies as they could get burnt (as it would need to be on full-blast), damage it, or cause a fire from the hay, so I'd say you would need multiple radiators. They would need to be thermostatic, I'd say you'd want house bunnies to be kept at 17c plus. Oil filled would be the safest in this instance I think, convector and fan are more of a fire risk and cost more to run when trying to heat a room. And you're looking at a LOT of electric - a 3kw heater will use 3kWh every hour if it is on full, that's about 42p every hour, plus the cost of the things themselves. And bear in mind putting the bunnies out there will likely trigger a moult too.
I can't believe that there is not a 4x3 little space somewhere in the house, next to the bed you'll be sleeping in perhaps, that they could fit in. A week in a dog crate isn't going to harm them, and in my opinion it'll be a lot less risky health-wise and you'll worry less. Didn't you buy an indoor cage for them? They could stay in that, it's only a week. Or take them to a trusted friend or other family member to watch them, and just give strict instructions on their care.
I don't mean to be negative, I just think it's better to be honest with you rather than hold my tongue and something bad happen.
