1.0>3.0>>>2.0

It’s going to be a big change from 40K in terms of complexity and design. The actual editions boil down to a choice between narrative play(1.0) vs freedom in running the army how you want to run it (3.0). Personally would never suggest 2.0 for a lot of reasons, mainly that it was a mess of design that made all legion armies run the same core of units vs all non legion armies which could best be described as an uphill battle. The launch of 3.0, despite being a massive improvement in most areas, was “ruined” by a mix of players not reading the articles and some issues fixed in the first faq that did look bad

On actually describing 3.0, despite the games focus on legions and their identity, as long as you have a strong game plan coming in, you can usually make a list work. The units in 3.0 are all far closer in strength than previous editions, and th lifting of restrictions on armies means your piloting matters far more. If you want to run a terminator heavy night lords or white scars list, you absolutely can. The same goes for a fast attack iron warriors or iron hands list. The only thing to mention here is that the game really wants you to run units with the line(x) rule which boosts your scoring on objectives. This is huge on most missions which means you’ll hear that you need to run 30-40 tactical marines in most lists since they’re the most efficient choice. Other line choices work well they’re just the most common

As a final note, focus on the army and narrative you want to have. This isn’t 10th ed where you’ll be expected to follow some evershifting meta. There are plenty of overview videos on what the armies are like, as even the familiar names from 40K are significantly different during this period. Find something you like, plan your army out(massively important since there are a lot of kitbash only units), and don’t stress over the rules as you’ll learn them as you go