About 40sqm then.  One butane heater would probably be adequate except during really cold spells when a second one might be a good idea.
Alternatively, a good log burner with a butane heater as backup would do the job.  The butanes are about 100 euros to buy but a decent quality log burner is going to be in the order of 1,000 euros.
If you have access to your own logs on site then it's an economic medium/long term solution.  If not, logs were in the region of 135 - 150 euros per ton last time I looked and you'd probably go through about 1.5 tons in the average winter.
Gas bottles are around 15 euros and last about three/four days (day and evening).