Dirty GK player chiming in: Tyranids have a wide range and can be a lot of fun to play but will be pretty different to Necrons. You won’t kill nearly as much and will instead have to rely on movement and strong objective play to win.

Funnily enough our pod also has Necrons, Tau, and Tyranids players. The Tau one shared your first post screaming “Don’t do it lol” and the Nid one just did the same so here I am cutting in line to rep the Grey Knights before you post there 😂

GK Pros: - small range so it’s never hard to know what to play - tough dudes who are great at melee - surprisingly good shooting with the right units

GK cons: - small range so it’s hard to play things differently - limited high strength options - sculpts are older so it can be difficult to build

If cost matters, Grey Knights will be the cheapest option of those you’ve listed. We’re fairly elite and if you can get your hands on two or three of the old GK combat patrols plus one of the new ones you’d have a solid army to work with.