Soldering is nothing like welding. Neither task is particularly hard, although welding takes a bit more coordination. Soldering is about as hard as using a hot glue gun or microwaving a meal, or scrubbing a toilet.
When you've never done it before, it is easy to build it up in your mind. Here's the things that matter:
acid core solder is only for pipes in a home
flux is important and the mess it makes is not
for just a small job, any soldering iron is fine, as is any solder
wet everything you're joining with flux and the solder will wick into place
the tip of the iron should be shiny with solder before you start, and this may involve a good bit of solder added to the tip and then removed by a wet sponge or wire ball made for the task
An adjustable iron is nice, and you'll likely find that eventually you will use it for threaded inserts in prints. There is a lot of marketing about irons and junk, but it is hard to beat the value of one of the Chinese 936 Hakko clone irons. Most of the marketing junk is to try and obfuscate the value and availability of these clones. The Hakko 900 series tips are the defacto standard and there are many extra accessory options available that are only possible with this tip/iron type. Last time I checked a 936 clone is usually under $40. The actual circuit board required to build one is under $5 on AliEx while the iron handle and lead are ~$8. You don't need this for a basic job, but an adjustable soldering iron is a lifetime useful tool to have on hand.
Good solder makes a big difference on bigger projects when you're doing this a lot. However, if I was in a zombie apocalypse, I could easily make a single solder connection by heating the tip of a screwdriver in a candle flame, use some resin from a pine tree, and a chip off of the pewter candlestick holder to solder a button to a circuit board.
Buttons can be a bit challenging with 3d printing design. It depends on your goals, but clearances and textures matter a lot more than it may first appear. It is possible to get something that just works, but is loose or crude. Getting a button like the inserts that go into a typical video game controller are quite challenging to clearance and develop a consistent tactile feel. I've done this in practice and it took a lot more iterations than I expected.