0.4 MM is the standard that most models are designed for. 0.6 MM will also work in many cases, but check the documentation to see if it’s compatible. 0.25 MM nozzle are great for fine detail on some parts, but are not needed unless specifically specified.
Questions specific to buying and maintaining 3D printers.
I use the E3D NozzleX. Brass Nozzles will work great with non abrasive filaments, but I highly recommend a hardened nozzle if you are going to print any filled materials. Stay away from low quality off brands if you can, a bad nozzle will insure a bad print.
There are a ton of options, your budget and goals will determine which you buy. I use the Prusa i3 MK3, a great machine, though a bit out dated by todays standards. The Ender 3 line of printers from Creality is extremely popular for a reason, because they are low cost and can work quite …
We use Prusa i3 MK3 printers for most parts. I also use an Ender 3.