The Core Rules were republished for Star Trek Adventures, as the
Star Trek Adventures Rules Digest and
The Klingon Empire Core Rulebook. Both of those are apparently reorganized for ease of use. I couldn't say what content has changed, but can say that I originally just owned the original STA Core Rules PDF and found it to be a confusing read and a challenge to build a PC. Whereas I own the soft cover of the Rules Digest, and my players and I haven't had issues with understanding and using it. Both of the new rules books feature black text on white, as opposed to the 1st books white on black.
The bigger issue of using STA 1st Ed, is finding Challenge Dice. It might not be a problem in scome countries, but in mine I had to order the online from a store on the other side of the country. You can instead quite easily substitue a standard d6 for them. They're only used for some aspects of combat and extended tasks, so it's not like you'd be too challenged often to use regular D6s. That said, my players and I find the CD with their Starfleet academy icons on some faces to nicely bring home the theme. It should probably be mentioned that the newer, 2nd Ed of the rules no longer uses Challenge Dice. So if you're just starting now, the newer rulebook is worth considering.
I have to say, the 1e sourcebooks, supplements and companions for STA are very good. I've spent hours pouring through the 4 different quadrant sourcebooks and enjoyed the time spent. I've even enjoyed reading the 3 division (Command, Operations & Science) supplemental books. I was fortunate to get every PDF Modiphius has published for STA 1e for about $25 USD in a Humble Bundle sale. So lots to read and GM with.
As to the other 2d20 games, there seems to be a consensus among 2d20 fans, that Fallout and Dune have benefited the most from what has come before. Were I not to have a group of friends that play TTRPGs and are also Star Trek fans, Dune would definitely be my choice. Read most of Herberts Dune verse novels in my youth, have watched the 2 movies and yes, I even enjoyed the TV series.