No. 1 is that you should totally do it no mater what. If you don't try it you'll never know.
No. 2 is that it's not easy to win but it's relatively easy to be competitive.
No. 3 is that if you show up at the start line and give all that you have you WIN even if you are last at the finish line.
Now to your your questions
1 and 2. Training is good no matter what. You do it because you like it and you will be doing it because you can get better/faster. Rest is even more important but rest with no big efforts does not do much. IMHO, running as a training is not that important. I do not train running at all and I do not like running but even in long sand running sections I am quite OK - think push, push, push. Roadies with no tech skills are very goodin CX and so are skilled MTB techies - if you combine both you will be at the top of things. Lack of power, endurance or skills will not reduce your enjoyment unless you are prone to be negative.
3. Observe, practice, race, make mistakes, race more, get better, get smooth and you will get faster.
Bike setup - all is good as long as it works for you. You might need to test few things but if you enjoy cycling you are up to some good times - get new things, change, adjust, replace, ... My favorite setup is 1X as I am a ww and it works great for me. 42 front and 11-28 rear. Disc hydro brakes are a big advantage and so are TA but you can get to top of the categories with rim brakes and QRs.
Tires - tubeless is the way to go IMHO. Many non tubeless rims can be easily converted and many non tubeless tires will work with the sealant. Play and try time. Mount them, get on some MTB trails and see how the rubbers work for you.
Additional note - you mention you don't do group rides. CX is kind of a a social sport. Yes, you race more or less solo but before and after the race it's all about having fun wit the group (teammates and competitors). Coming to the race solo, racing and leaving right after will make you miss lots of fun. It's worth getting social IMHO.
Have fun mate!
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