Perhaps putting the phrase first might stress the point more. The word "surplus" should be plural. Here is a suggestion. When governments have budget surpluses, I believe that we should prioritize first, on investment in the infrastructure and then on funding the arts. Hope this helps

i believe that our government (anyway you are talking about the government) should make priority on investing in infrastructure first, then they can allocate the fund on arts if there is a budget surplus available. English is my country's second official language.. hope anyone can correct me if there is a mistake

Pretty close and didn't sound awkward. I believe that we should prioritize investing in infrastructure and then fund the arts when governments have a budget surplus.