Hey Phly,
I think you're reading into Macs being better in the design fields (or in any field) a bit too much... It comes down to specs, software availability, support & resources for the software you're using, OS preference and ultimately affordability. That final one is the deal breaker and you said you're on a budget.
Do some research and you will find when it comes to bang for buck Macs simply cannot compare, without knowing your actual budget i can assume you would be getting a far better machine were you to go for a Windows alternative. I'm not being biased, i own and love a 2014 MBP its my daily driver. Think you might also find if you research the importance of PPI in graphic design the 2012 13 Inch Mac you're interested in which only rocks a res of 1280 x 800 might not be such a good choice (PPI of 113)... A high res external monitor is always better to work on but you cant exactly take that to your clients or use it on the go. Not to mention you would be powering it with an Intel HD 4/5000.
On the subject of GPU: Most of the major design apps these days utilize the GPU for a lot of their processing tasks and depending on your field you may want much more than the HD 4/5000 can muster.
tl;dr Think the 2012 13 Inch Mac would be a bad choice for graphic design. We're in 2016! 4 years in tech is a very long time.