I see dko22 beat me to a response so there is some duplication here but hopefully also some useful information!
One of your questions relates to an appropriate sound card or audio interface. Assuming your focus is on using sound libraries to make your music “sound right,” even relatively inexpensive sound cards focussed on use by musicians rather than just listeners should work fine. More expensive audio interfaces (sound cards) will have multiple inputs for microphones and midi (electronic) instruments but these are unnecessary and will remain unused if your sounds are coming from sound libraries on your computer or a connected drive.
As you suggest, more RAM will allow you to have more sounds loaded and ready for immediate access by your computer when it plays back your music. However, the amount of RAM needed will vary depending on the particular sound library you are using, the number of instruments in your composition, and the number of articulations used by each instrument. In general terms, the amount of RAM required for a vocal piece accompanied by piano or guitar will be much lower than that required for something like a full symphony orchestra. I am sure many musicians manage to get by with 8 or 16 GB of RAM but this will vary according to the type of music they are writing and the libraries they use and there is little advantage in having more RAM than you need other than being ready for “larger” projects in the future. Most sound libraries include information on the amount of recommended RAM.
If you will be working on lengthy projects with large numbers of instruments, the speed and number of CPU cores in your computer will impact how comfortably it handles the playback and notation aspects of your Dorico projects. I use a 2013 MacBook Pro with a 4-core processor running at 2.7 GHz and have no difficulty with even orchestral projects, although most of those max out at about 200 bars long. In short, my understanding both from my own experience and from reading this forum is that many users are comfortable using Dorico on a computer with 4 or more cores running at something comfortably over 2 GHz, although professional musicians and composers as well as others writing for large numbers of instruments often use even more.
The goal of getting computer-generated music to “sound right” has at least two different focusses. One of these is the beauty and, if we are talking about traditional instruments, the authenticity of the sound produced over our speakers or headphones. Varying personal taste plays a significant part in such decisions and different people will prefer the sounds of different sound libraries. The second variable is the extent to which the phrasing of the music (including timing and dynamics issues) sounds like it might have been played by human musicians. This is the more difficult thing to get right, although different notation programs including Dorico allow users to choose a level of variability in the timing of note attack and conclusion that more accurately reflects real human players. As well, many sound libraries are designed to play a series of fast repeated notes in “round robin” fashion which translates into varying volumes and durations for each note in order to avoid a “machine-gun” effect. In addition, Dorico has a piano-roll feature in play mode that allows users to adjust the sounding duration and volume of individual notes without altering what appears in the notation and this can add to the believability of the playback.
Almost certainly, the most effective sound library in terms of producing human-like phrasing is NotePerformer which is specifically designed for use in notation programs, as opposed to Digital Audio Workstations like Cubase, Logic or Pro Tools. On the downside, the beauty and authenticity of the instrumental sounds isn’t the best but this is balanced by modest cost, low RAM requirements, fast loading times and very believable phrasing as well as decent sound when playing with a large number of instruments that reveals less detailed exposure to individual instruments. However, NotePerformer is primarily focussed on orchestral instruments and might not be a first choice if you want to write for something different. Nevertheless, for anybody who has some interest in writing for the included instruments (probably the widest range of any instrument library) my personal recommendation for a first instrument library would definitely be NotePerformer.
If you have further questions, it would be helpful to know what kind of music you are interested in writing. I hope this is helpful.