Hey Cold Guy, welcome to the Forum!
I feel like what you might be asking is more along the lines of 'which units should I avoid buying?' If this is the case I think we can answer that in the following:
Don't Take:
Don't make a Troglodon (Carno/Trog Kit) - He just generally doesn't work. For what you pay for in points, you can add redundancy elsewhere, and achieve many of the same things he can (with the exception of the roar) elsewhere. For a model that costs as much as it does ($), make it a Carnosaur, or don't buy the kit.
Chakax - I still don't understand this guy. Like 2x the points that he should be.
Tiq'Taq'To - His abilities are weirdish and suggest that the authors meant to override some BRB rules, but failed to directly do so - avoid him for now, and see what an FAQ or 9th ed. brings.
You can take, but it's sub-optimal and will often leave you frustrated:
The Carnosaur (Oldblood or Scar Vet versions). While an awesome kit and model, you will find that his stats just don't quite add up where they need to, especially when facing the most common opponents (Elves, Warriors, and Daemons). Just a touch too expensive for what he does, and really lacking in some important offensive and defensive stats (WS 3 makes me sad).
Swarm-making Bastiladon - you will find that typically the Solar Engine is a waay better choice (and worth making the kit that way, or magnetizing it at worst). For the points, a model that makes additional swarms on a 4+, you could just buy 4 more swarms. His offensive stats aren't good enough to make him worth that ability, typically.
Blowpipe option on your Skink Skirmishers - just try it out before you build them that way. You should very quickly see that Javelins are the way to go.
Spears on Saurus - Whilst Saurus are often considered a sub-optimal choice in general (I field 1 unit of 25 to good effect), the Spear upgrade typically isn't worth the additional attacks. The Parry save from the shields will help you grind your opponents better, in a typical engagement.
Less necessary because your Core already fills this role
Razordons - I think this is a very good unit, honestly - but your core choice of Javelin Skinks can do mostly the same job. In reality, the Razordons add the Close Combat punch after the shooting. You can definitely make this unit work with magical support, and it's decent without it, but I would just put it lower on your purchase list.
With work and practice, I think players can make everything else in the Lizard list work. Some are easier or harder than others, dependent on the play-style you develop. Hopefully this will save you a bit of kit buying waste.