Can i finance an Iphone 7 and be able to activate it on my current metro pcs line?

MetroPCS does appear to support the iPhone 7.

http://www.apple.com/iphone/LTE/

Be certain you purchase the Sprint / Verizon variety of iPhone as explained on the page I attached above.

But regarding paying in installments, all the installment options on the Apple Store say "Works with your carrier ...". Yet the only carriers that work directly with Apple in the U.S. are AT&T, Sprint, Verizon and T-Mobile. So it appears you are out of luck regarding installments. You can ask MetroPCS if they would sell you an iPhone using installments. And, of course, you would have to pass a credit check.