Do not sell stock images with the idea that you will become an overnight success. You might become rich quickly, but it's unlikely due to all of the competition that you will be up against (millions of images).
Your success will come down to your skill level, and the type of images you are selling. This type of business will usually provide enough income for you to upgrade your equipment, and
hopefully, with time, your income will increase to more comfortable levels. But there are no guarantees.
The latest and greatest versions of software programs are probably not necessary. If you're just a pure photographer, then just buy Adobe Photoshop Elements or a free alternative such as GIMP. You don't need Adobe Photoshop C$. And since stock agencies want Illustrators to dumb their files down to an older EPS format anyway, there is no need to buy the latest copy of Illustrator (insert number). Just make sure it's not a pirated copy.
Ensure that the agency you are submitting images to allows for you to easily delete your images. Some agencies require you to wait a certain amount of time, and some require you to email someone from that agency in order for the images to be removed. It can be a pain to do this. The only catch is that EF lenses may cost a bit more.
Quality always defeats quantity when it comes to stock imagery. But if possible, have both. And focus on quality.
Do not log in to your account to check your earnings too often; it will drive you nuts. Limit yourself to once per day or maybe just every other day.
You may want to avoid buying EF-S lenses if you plan on using a full-frame camera in the future. Instead, purchase EF lenses; they will work on full-frame cameras, and these lenses will also function on crop body cameras.
Organizing your files (when possible), and backing all of your work up is very important. An external server that holds a couple of hard drives is a good idea.