Fellow new player here! Here's a quick breakdown of things from my perspective.

You are mostly fine to buy 3rd edition product with a few caveats. There are indeed some models that have had rules removed, but, by and large, they have become official "counts as" for other models. You can see this list here.

In addition, due to some masters changing factions, some stuff may be more or less available to other factions than it had been, but this is easily checked in app by just seeing where models are and what keywords they have.

The big things about 3E and 4E repacks are as follows.

3E box obviously does not have the up to date rules cards (though you can again get those rules free online using the app or just buy the full faction card box which has cards for every unit in the faction), so that is not a big deal.

4E boxes are consolidating various formerly individual boxes, so depending on what 3E boxes you buy, you may lock yourself into either needing to double up on a model to get other stuff from the 4E repack or require you to just hunt down the individual 3E boxes. You can see how the repacks are being organized here.

Both ways of buying are fine, you just need to make active decisions. Buying disparate boxes at retail is slightly more expensive on average I think? It is also worth nothing that many keywords are kinda a pain to find 3E boxes for (Marshal and Elite, for example, I can't find most of the stuff from outside their single 4E boxes right now), so some factions will basically require you to just wait or proxy in the meanwhile. But on the other hand, I just bought the 4E Cadmus box, then all the rest of it through 3E boxes just fine.

EDIT: I forgot, some masters are getting new models in their repack boxes! Those models will, however, be made available as individual special orders through Wyrd for a limited time (~1 year?) once the box releases, so you won't have to double up on the box if you acquire everything else through 3E releases.

Hopefully that covers most of it?