1) youtube has loads of videos about safety kit and slicing parameters that are important.
2) I don't do design as such, but I use Meshmixer and 3D Builder as my go-to digital kit bash tools. For example, weapon swap to turn a fantasy lioness lady into a sci-fi version
3) What kind of stuff? For printing consumables, I go to Amazon (gloves, filters, IPA, resin). For models I mostly browse MyMiniFactory and Thingiverse. If I have a particular need, then a google image search is usually quite fruitful. Thre are search aggregators for STL files like Yeggi and STLFinder that are quite useful. Cults3D is quite a good site as well.
4)
Napoleonic
https://napoleonicstlfiles.com/en/10-stl-files
https://www.myminifactory.com/search/?query=napoleonic
WW2 and Cold War
Thingiverse has loads of free options
https://www.thingiverse.com/thing:4388227
https://www.thingiverse.com/thing:3553160
https://www.thingiverse.com/thing:2476519
There have been several kickstarters recently with cold was stuff as well, e.g.
https://www.kickstarter.com/projects/bm3d/british-army-of-the-rhine-baor-3d-stl-printable-vehicles?ref=discovery&term=british%20army%20on%20the%20rhine
They are closed now, but you might be bale to find the vendor online and buy the STL models separately.