Re: About to get started with reloading...
Welcome to the fold! Your Lee press shouold be no problem at all for the 7mm STW, and of course the 300 Blackout will be a piece of cake as far as the sizing goes. You might want to read up a bit on the specialized techniques for loading semi-autos, if you're using the Blackout in an AR, but with this one you'll still need to take it a step further. This is especially true if you're loading subsonic rounds for use with a suppressor. AAC has some pretty good tips on the care and feeding of suppressed rifles, and a great deal of info on the Blackout. Worthwhile stuff.
Spilled powder, no problem; broom and dustpan. I keep a bench brush on hand just for this type of mishap. Just have to avoid the vacuum cleaner, as it's not a good mix with propellants. (Dave Barry once wrote a column about an annual competition between vacuum cleaners siphoning gasoline, and just how fast they turn into jet engines. Hey, guys will be guys!)