If cost is a concern, use Klip gut with a smooth poly. TW has a hybrid for $22 or you can build your own by buying a poly and a package of hit for a little less. You can get the cost down to about $19 a set.
I STRONGLY recommend using gut/poly combo due the comfort, power, and control mix that it provides. Also, this setup easily lasts 2-3 times as long as a full poly bed and 4 times as long as a full bed of multi or syn gut. This, from a price perspective, it’s also a good value.
if you do want to use a full poly, string at 50 or less, and use 16g due to the openness of the string pattern. I’ve found that ProLine X works very well in this frame and offers better control than ALU Power in this frame.