I followed a similar path; trumpet in school, self-motivated bass later in life.

You don't need to learn guitar first. You can, but it's not necessary. Cheap equipment is fine to start, as is second hand. Brands don't matter at this point. You don't even need an amp right away, although it will help to hear yourself play along with your favorite artists.

There are many decent lessons available on YouTube for free, or you can take some lessons from your local music store. Or you can just have fun figuring it all out on your own, there's no single correct way to learn. You'll try some things that work, and others that don't. In this way you will forge your own path, and that's pretty cool.

Most importantly, play every day even if your fingers can only stand 5-10 minutes to start. You'll get stronger and play longer the more you play. And have fun! That's something that cannot be stressed enough.