You might not want to focus too much on getting a really fast microSD card and instead the capacity. When using a card over 32GB, the largest your phone will support, is going to be risky thing to do as you'll never be sure about its reliability.
Also, given your phone's dated hardware specs, a really fast card is a bit of overkill and the system resources won't be robust enough to even take advantage of the faster read/write aspect.