HI and welcome,
As for the wood floor, I wouldn't have thought that would have come from the sketchup file, but then again I don't use SKP, it may not necessarily be an object It could be a single facet, for 3D printing it doesn't matter about what a materials are set so the warning can be ignored, if you could post the file I'm sure someone will take a look.
As for Printing, it may not turn out as expected if you are printing as a completed piece, the problem would be support material for the spokes to stop them sagging when printing, it can be done but my need some thought as to how it would be supported before final printing, some printers work differently and could do it no problem (but some of them use lasers), personally if I was printing it, I'd slice it in two vertically, i.e. so each half has - half spokes, hub, rim, and glue the two halves together, but that's just my own opinion, maybe other would comment on how they would do it.
## Edit ## Henry posted while I was slowly typing
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