Like @Arty_Marty I do most of my design work with Fusion 360. You can see some of my stuff here (including some gel blaster bits). You can get decent result with cheaper printers too—I use a Monoprice Select Mini (MPSM), which is about the cheapest non-kit (i.e. build it yourself), non-proprietary filament (I’m looking at you Da Vinci) 3D printer you can get with a heated bed.
I print exclusively with PETG, which while not quite a strong a ABS or Nylon, is pretty close and doesn’t suffer from the start-drooping-on-a-car-dash issues that PLA has. I’ve printed structural parts (plates to mount LED blinkers on my motorbike) which survived summer heat in central Queensland with no issues.
While you can (very easily) fall down the rabbit whole of printer upgrades (sound familiar?), you can still get good enough results without doing so (unless you buy a kit—they really are a bottomless pit of tweak—with some exceptions… Prusa).
So short(ish) answers to your original questions:
- Design your own stuff using Autodesk’s Fusion 360 (its free, and pretty easy to use if you have any previous CAD or 3D modeling experience, and well set up to produce files for printing)—there’s other options for more novice users, but I don’t have any experience with them (e.g. TinkerCAD)
- Get already designed files from Thingiverse—most designers just give you an .stl file (3D model) of the objects—this means you can print them, but its hard to modify them. Others (like me!) also provide the design files (files types differ depending on the design software used—Fusion 360 files are usually .f3d files)
- To print an object you need to run the .stl file through a “slicer” (I use PrusaSlic3r)—this is software that converts the 3D object into machine code (called g-code) which tell a printer "move here, extrude this much filament, at this temperature etc.). Getting the g-code (which is specifically adapted to the settings needed for your printer, filament type etc.) onto the printer is what allows you to print—usually you put it on an SD card and insert it into the printer (some also support USB connections to computers, network links etc.)
- As for which are the best machines… how much money do you have (and how tech savvy are you)? I quite like my little MPSM—I think I picked it up on sale with free shipping from Amazon for ~US$150. Some of the ones that Aldi has on special buys are half decent (the ones with heated beds)—I have a heavily modified one of these I picked up second hand for $200 (though, per comments above, I still haven’t used it in the 6 months I’ve had it—needs calibration and setup). It you want something better, a genuine Prusa i3 will set you back ~$800–$1100 (depending if you go kit or pre-assembled)—these are apparently very good for that price point. Anything else that’s better is generally $1500+, though I haven’t really looked for a while.
Hope that helps somewhat!
edited to fix ‘midnight’ grammar and spelling