Lots of questions, so I'll chime in:
There's really no such thing as a "bad" choice for a kit, though I would say there are worse options. A full history would take too long, so the abridged version: Bandai's older kits (pre-2000 or so) are typically not so great, nearly always requiring glue for assembly and paint to look even remotely accurate, and with questionable proportions and posability. Their more modern lines, like High Grade, Master Grade, and Perfect Grade (with Real Grade coming later), are much better in comparison, giving you a color-accurate model that just snap-fits together. Typically, the newer the kit, the better the tech behind it will be, but you typically can't go wrong as long as you see one of those acronyms on the box somewhere (HG, MG, PG, RG).
As an example, here's what the Guncannon you bought looks like out of the box, compared to its later High Grade versions (original on the left, first HG in the middle, updated HG on the right):
But hey, you did say you bought it for painting practice, so you'll definitely be getting your money's worth there.
The most important thing is whether the mobile suit in question looks interesting to you. It doesn't matter if the build experience is amazing if you're left with a kit you don't really want to look at on your shelf (and honestly, most kits are going to feel about the same to build, even among different grades). For ideas of what to keep an eye out for, the best resource is
Dalong's website, who has spent over twenty years building and documenting literally thousands of gunpla. He may not have reviews up of the absolute latest kits (GQuuuuuuX kits have only just started coming out, for instance), but he has more than enough to get started. Scroll through his listings and see what catches your eye; it'll also give you an idea of what to expect from some of the kits you've already bought.
Also, when you get home, look into picking up an Entry Grade to practice on. They're simple and cheap kits that don't even require tools to remove parts from the runners, and they're great for practicing painting and detailing.
And I'm pretty sure you can find files for 3D printing stands somewhere, though I haven't really looked into them. Most modern kits will have some kind of hole that an Action Base will have some kind of an adapter for, and those that don't can be attached some other way. Some even come with their own adapters and simply require you to provide the base. I also know you can pick up third-party bases that are usually cheaper, though I can't attest to their quality.
At any rate, hope you enjoy your builds!
