If they sign him to a long term contract, then they're not going to be able to afford a second line center IMO...at least without weakening other areas (for instance moving both Chytil and Lindgren).
More likely I'd expect a two year deal for Kakko, which would carry the Rangers through the worst two years of Cap pain, but could then cost them dearly down the road. With the current expectation that the Salary Cap will significantly rise for the 2025-26 season, the Rangers are likely to be making some hard choices in the next three seasons.
Chytil, Jones, Lafreniere and Miller all are due contracts at the end of next season. Lindgren and Schneider the year after. Kreider and Trouba move off a full no-move for the 2024-25 seasons, which could give the Rangers a couple more options for that season, but ultimately some of these guys aren't going to be playing out their contracts or getting moved.
The biggest problem for the Rangers is that they need some cheap players who can play in their line-up to balance off the huge contracts that Panarin, Trouba Zibanejad and Fox have, and to a lesser extent Kreider and Shesterkin (plus Copp or Strome if they were to resign either).
Kakko potentially fits that profile, but it depends on him finding his game and becoming a top six contributor. Same goes for Lafreniere. The advantage Kakko would seemingly have, is that he's a right wing on a team with lack of depth on that spot, with the top forward prospects at this point being Othmann (LW) and to a lesser extent Will Cuylle (LW) and Ryder Korczak (C).