How good is 1. e3 2. c4 as an opening for White? How good is 1. e3 2. c4 as an opening for White? Afterward, developing the knight on b1 to c3, and developing the bishops too, it seems good. Castling is preferably done on opposite sides as well.
Overall, I wouldn't suggest it, especially for a beginner. There are better ways to fight for an advantage.
By playing e3 first, you are allowing your opponent to do almost whatever they like on their first move. Are you OK with them playing 1...d5? Is that going to prevent you from playing c4? If that's going to be a problem, you may want to play 1.c4 first instead (which is the much more reputable English opening.)
With a pawn on e3, your c1 bishop has limited scope. Perhaps you want to fianchetto it to b2? Otherwise, it's either stuck on d2 where it doesn't do much, or you need to play e4 to free it, which perhaps means that e3 was a waste of a tempo.
Castling on opposite sides seems like it's something you need to decide later - don't do it if you aren't in the right position. If your opponent castles kingside (which they usually do), then castling queenside means that you're putting your king on the c-file, behind a pawn you've moved two squares (and if you've fianchettoed the c1 bishop, the b-pawn has also moved.) Depending on what your opponent has been doing and how open the game has become, this may be a bad idea.