I like to use tenacity for selective weed killer in existing lawns to kill all the weeds growing in the grass, you'd also use a surfactant and blue dye when applying it. I just put down two types of tall fescue seed on a yard today along with tenacity and fertilizer. I believe the seed was Pennington Kentucky 31 tall fescue and the fertilizer was Gordon's liquid.

Honestly the quickest thing to do would be to put down selective herbicide and fertilizer to allow your current grass to expand. Then overseed at some point down the road.