At 500 points, play Combat Patrol and bypass the usual
FoC rules.
Any Crisis Suits you buy, either pay to have them magnetized or do it yourself (I now recommend paying pro modelers or more talented buddies over my own poor efforts).
Start with a box (2 of these actually):
https://www.games-workshop.com/en-US/Munitorum-Containers-Tau-Empire
Over the years the box deals have been okay to great. This one is okay (for the terrain pieces).
So, Crisis Suits, minimum amount of Fire Warriors (Pulse Rilfes). Once passed 500 points, this is good:
5 or 6 Suits, one 'Vre with
MSS and C&CN, give the rest dual PRs and maybe one or two dual
FB suits. Add on the Irid Commander, dual PRs on him, a couple of Gun Drones and that unit can kill 90% of
40k's units in one salvo.
Riptides. 3. Ripwing. HBCs.
SMS.
Then, the tetras mentioned above are good, along with a b-side team, full 3 suits and their 6 Missile Drones. I am mixed on the Ghostkeels and their mechanics, having killed them in most games verus a Tau opponent. If you have the money, a StormSurge when you head into points beyond 1500.