I'm trying to get back into HAM after 30 years out of it. Starting with meshtastic let's me play around without spending a lot on hardware, and before I get my license back.
There are great prebuilt nodes for under $50, and as low as $20 if you build your own from parts.
No worries! If you're starting out you can't go wrong with a T1000-E. It's water resistant, has GPS built-in, and ties everything together in a neat package for ~$35. There are plenty of other nodes out there but it's more of a build your own kit situation. If you're into microcontroller programming and stuff like arduino/RP/etc then Heltec v3 Lora boards are good tinker hardware.
We also have a community here on mander for meshtastic.
Yes! If you leave the default channel enabled it will join the public meshtastic network. But you can run eight channels at once, and the others can have different encryption keys.
The default setting is also to forward any received packets to the mesh, even ones it can't decrypt.
High power radios in the hands of people who don't know what they're doing can cause a lot of issues. FRS/GMRS radios under 2 Watts in the U.S. don't need a license. Licensing is a way to make sure people using them know how to do so effectively, and provides for stiffer penalties if someone unlicensed causes issues.
You could also get one of the SSD drives that look like flash storage. I got a "usb stick" with 512GB SSD for about $60 last year.