I actually recommend an older laptop (i.e a Thinkpad X61, Thinkpad X200). These will be fine of pretty much any 2D game (software or hardware rendered) and have the benefit of being very compatible with most operating systems, including Linux and BSD if you wanted to prepare yourself for future platforms such as Steam OS etc...
They also provide a very smooth way of developing (development libraries as part of the package manager) and additional tools such as Valgrind which is fantastic for memory debugging when dealing with native languages like C or C++.
Another alternative I might also suggest is a cheaper / older MacBook so you can test your game out on all 3 of the main platforms, Windows, Mac OS X and Linux. Installing Mac OS X on a non Apple machine is touch and go... Especially for laptops.
Plus older laptops which are less performant are a good way to test your games on slower machines... They also dont overheat your crotch when using it as an actual "laptop" ;)
You can pick up an older Thinkpad for about £70