MaltYebisu

This is important. You should add a line or two about it in the first post. I was naive enough to think that kOS would not be able to do any harm outside of KSP. There are potentially a lot of people like me who thought the worst thing that could happen with a kOS script was a the game crashing. Perhaps a link to, or an explanation, how people can set up a VM to do "Twitch Plays kOS" stream safely would be nice to add in the docs or the first post.

I can't wait to try it out

I suggest adding an experiment to all probe cores to avoid KSC science hopping in the beginning. RP-0 have telemetry report/ analysis, and it helps a lot. If you make flying low and space low biome dependent, then gathering science with sounding rockets becomes an alternative to KSC science. I also would move batteries to the first node. There is not enough electrical charge in the Stayputnik to send science, and that makes the first flight a science only or a flight only mission. For me, the idea of putting something on the launchpad just to measure the temperature and barometric pressure is not a fun way to begin a space career. I think reaching space is a little bit too easy, and I was able to reach it without even spin stabilising using only three fins on my first (real) launch. I didn't install FAR, but I still think reaching space should be harder than that. Maybe the small liquid engine and Oscar-B tanks should be moved and the first solid-fuel booster could be made less effective by making it a bit heavier or something.

If you are balancing the science and tech-stuff, then I hope you find a way to make the early game possible even on hard without the KSC science hopping.

Wow! Congratulations on the release! I want to try it right away

Principia is a lot of fun. I had to learn how to do rendezvous again when I tried it, it was great! The only problem for me is that it was too demanding for my computer. This might be because I use a Mac, but 2.3GHz and 8GB RAM is not enough for the Principia mod. Everything works except timewarp being too slow to do any interplanetary missions. If you try Principia, you need to start a new game, because it moves the planets around. You might also want to disable astroids to make timewarp a bit faster. Principia Wiki - Installing & FAQ I recommend just starting playing RP-1. It is easy to install for 1.3.1 on CKAN (RP-1 Wiki - Installation) or if you want to try it for 1.4.5 then you should have a look at the KSP 1.4 Golden Spreadsheet. I agree with @hypervelocity! RP-1 and Principia is like playing a new game. It is fun.

There are also a lot of duplicate engine and pod-parts if you install different part-mods. It would be nice to have a convenient way to prune duplicates and unwanted parts to make KSP run smoother and clean up clutter.

It's beautiful, but you need a powerful computer to play with visual mods. (At least mine is not powerful enough) If clouds become stock, they need to make it a setting you can turn on or off. I hope they do though. Maybe they will find a way to make it less demanding on the system.

I think it's essential to keep KSP a fun game where realism takes a step back to gameplay. I believe leaving a lot of realism to mods is a great way to get the best of both worlds. Never the less, there are still some things I think should become stock. For me the most important one is persistent rotation during time-warp. I thought my game was bugged after I managed to reach orbit for the first time and the time-warp killed all rotation in the craft. It bothered me because time-warp is supposed to be a way to fast forward time to make the game playable, not something that exists in the game world. Because it is impossible to play the game without switching vessels or time-warping, it is not even something you can refrain from doing to avoid cheating. I know changing this now would perhaps force a lot of people to change their play style a bit. But adding a kill all rotation button in the debug/cheat-menu would be a better solution in my opinion. I'm not familiar with the technical reasons for killing all rotation during warp, but if the Persistent Rotation Mod can manage rotation with little to no performance issues, then implementing it in stock would not be too hard I imagine.

Here it is: KSP 1.4 Golden Spreadsheet

Congratulations with the release! The new version deserves a new thread. RP-0 and RP-1 are different enough to warrant it, besides, some people might need the information in the OP for a 1.2.2 RP-0 install. Having two treads might make it easier to find what they are looking for.

@Epox75 I used The wayback machine to find the dead Better Than Starting Manned thread and managed to download the mod from the links in the OP. Maybe you will find the dead thread on there.

RP-1 is not on CKAN because it's not released yet. You have to download the developmental branch of RP-0 on GitHub. The best way to install it is to follow @Bornholio's install order in the Golden Spreadsheet for 1.3.1. Some of the required mods are on CKAN but some you have to download manually.

There are a lot of improvements "under the hood" and many small changes, but as a player, the greatest differences I noticed are: Tooling cost: Many procedural parts have a "tooling cost" to compensate for the lack of "part purchase". If you don't tool your tanks the production cost and time will go up. The tooling cost changes the way you design your rockets because you would usually try to re-use the same size of tanks to save on tooling costs. Kerbal training: You need to train your kerbals for the different capsules you are using. Kerbal retirement: All kerbals have a retirement date. This date can be extended by letting your kerbals fly missions. Because I don't use planes (and a lack of planning), all 4 of my original kerbals retired before I could use them on my first RP-1 game... I think these changes add to the challenge, and it would feel a little strange to go back to 1.2.2 RP-0 after playing 1.3.1 RP-1. It takes some time to get used to the new gaming mechanics, but I ended up loving RP-1. If you want to get a feel for RP-1, then I recommend @NathanKell's "Rusty RP-1" series on Youtube. It's a bit old, so some things have changed after he recorded it, but it's an excellent introduction to RP-1, and he explains some of the changes and the reasoning behind them.

I love and share your optimism! I own KSP on GOG, and they don't have the 1.3.1.1891 version available for download anymore, only the 1.3.1.1836 pre-release version is available on the "pre-release beta channel". If you contact customer support, they will let you download any version you want, but I can imagine that this is a barrier for most GOG users of KSP.