Not using soap on cast iron is a vestige from when soap had lye in it. Modern soap, even dish soap is good to go on cast iron. It will remove the oil that has not yet polymerized, but that isn’t your seasoning. The seasoning is oil that has polymerized, and sodium hydroxide (lye) is capable of depolymerizing those oils and messing up your seasoning. Modern soaps won’t/can’t.
Even so, cast iron really doesn’t need much washing if any. Scrape the tarnation out of it with a straight edge spatula, scrub with salt if needed, fling everything out, wipe down with oil and admire your work.
I would not buy a modern pan. The modern manufacturing process is such that the surface is much more rough, even when seasoned. Almost as if they pour it into a mold, let it cool, and slap a label on it. Old cast iron used to be polished smooth, and you can still find it for dirt cheap, or worst case scenario you pay the price of a new Lodge but you get a superior product.