Everything I know about McDonald is what you can read at the link, where Ben Badler lays it out. He's lefthanded, profiles as a corner outfielder, is "toolsy" with great plate discipline and questions about his power.
The Rangers have not come close to maxing out what they are allowed to spend on the international class that is eligible to be signed from July 2, 2012, to July 1, 2013, with Jon Daniels saying that the bulk of the money the team had budgeted for internally (as compared to what they are allowed to spend under the new CBA) for 2012 was used on Jairo Beras, who, of course, doesn't count towards the 2012 J-2 class.
So this is a situation where they had money they could still spend under the cap, and decided to use some of it to grab a guy they seem to think is intriguing.