Mostly you have to buy this new.
Now voltage from battery research (Part C) takes account.
Solar panel voltage needs to be above.
If having 12V battery, go for 18V solar panel at load current, but not "non load voltage" (is 20..24V for 18V) panel.
If having 24V go for 32V etc.
Mainly care for not getting cheated by non load voltage.
(General schematics: https://mstdn.social/@hamjb/103182013100915068)
What about power?
Like battery: bigger is better.
What is important: panel should look into sun directly, south is perfect, but even east or west direction works (but gives less hours output a day).
Extremely important: never have shadow from buildings around or trees etc on panel.
This cuts of power tremendously.
If you can't avoid, place panels in parallel for having most of them without shadow some time.
Another good news on panels: you can start small, having one first.
If getting next one (same voltage) you can attach later.
Please care if sum of currents are fine for step down (Part B).
In case of "wow, that much current, would break my step down": no problem at all. Do it like picture and split using more converters.
Any combination possible like your needs.
Diodes (schottky recommended with low drop) required only, if not within converter. My recommended type doesn't have.
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