You are absolutely right. RNG k-e model can be used for a wide variety of flow regimes. SST k-omega, which is a blend between regular k-omega and k-e, can also be accurately applied for a wide range of Re.

Having said that, since you are working with a heat transfer problem, it is extremely important to resolve your sub-layers. Please note that it is important to resolve both momentum and thermal sub-layers in your case. Therefore, most meshes with y+ < 1 might be necessary. As mentioned by Amine in the earlier post, you might have to take the effect of Pr on your meshes when modeling heat transfer. If Pr > 1, your thermal sub-layer is much thinner than the momentum and the problem will require much thinner meshes. Please follow Amine's comments about ensuring 10-15 layers for the sub-layer.

Because of this requirement, SST k-omega, rather than RNG k-e with standard wall function, is more suitable for your problem.

Hope this answers your question.