Long-time Doha resident here: By that time of year, even at 9am it will be hot and the roads in that area aren't the easiest for pedestrians- there's still a lot of road development and missing footpaths. The taxis from the airport are well-regulated and you can ask for a taxi that accepts cards. You can also take cash out from a number of atms between arrivals and the taxi rank. And the taxi trip won't be very expensive. I would definitely take a taxi.