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Looking through my codex it says I have access to the Predator, Vindicator and Whirlwind but these are only available to Space Marines by default (meaning I have to buy the space marine stuff). Just wanna know if this is the correct way to go about getting units that are not strictly DA but aren't made into DA units?

Someone said just paint them but I want to be sure.

umm...what? the DA codex contains the rules for Pred, Vindie and Whirlwind, so no need to buy a SM codex.

This used to be the case with the pre-previous DA codex

Paint them as DA use them as an allied detachment.

In the DA 6th ed codex they have all of those tanks you mentioned.
DA are Space Marines. So buy the kits you need that are for all space marines.
If you want to DA them up to be even more special, you can get the chapter specific doors and emblems from Forgeworld.com that is a GW subsidiary.
Or get good at detail painting yourself.

And welcome to the 40k hobby.