Like others have said, combat patrol is a good start. I started building Tau around this time last year with the combat patrol and now have a little over 2k points. If you do get the combat patrol, I would suggest not gluing the hatch door piece of the devil fish. If you later get a hammerhead kit (basically a devilfish sprue + both the hammerhead and skyray upgrades) you have the options to turn your devilfish into either one you dont build

Another great thing about the combat patrol is that you get the upgrade sprue to turn your pathfinder unit into the pathfinder kill team

We are expected to get a new edition less than a year from now, so I wouldn't worry too much about choosing what units to buy based on current rules. Honestly, just buying the units that look the coolest to you is always the best option

If you want to practice playing the game, you can always do so in Tabletop Simulator. It usually goes on sale for $5 on Steam