Input from a novice so take it as you will... If youre questioning how hard it is or how good you have to be to flatten a board with a hand plane vs a machine, dont. My assumption was that it was an "old school" view but quickly realized how easy it is (being slow and deliberate and checking as you go of course)While you can certainly get by with surfaced lumber, the issue there is finding straight boards and the fact that anything you do to straighten them will reduce the thickness.
Personally, I think the first ”planer” you buy should be a hand plane. I would start with a #4 and #6. You can flatten any board with hand planes.
Another option is stick with surfaced lumber and buy an open end drum sander. 😉
I see a lot of resawing rough stock in my near future for some projects and its tough (for me) to be really good at that. So boards will definitely need to be thicknessed. My first instinct was to look for a thickness planer but I have since reconsidered. A good hand plane, sharpened and a drum sander (I plan on making from some youtube plans) will do the trick.
The jointer, to me, is still invaluable. Although with proper jigs Im sure handplanes can get just as perfect a jointed side. Im just not there yet.