First advice would be to get the beta version of 0.13 instead of the current public release (0.12.3). 0.13 could be out soon (Tommaso has tweeted that build 5 may be the last for beta testing). 0.13 adds some basic redstone physics (no machines, repeaters, hoppers or pistons yet). For more on the getting the beta builds of 0.13 go to the official blog: http://pocketbeta.minecraft.net/
BlockLauncher pocketinveditor and MCPE Master are popular editing tools in Android.
I strongly recommend making frequent backups of your worlds especially if you are using mods. On android they will be in /sdcard/games/com.mojang/minecraftWorlds and each world is a folder with a random hash for a name. The world name is saved in levelname.txt so finding which world to backup is usually easiest by searching for the world name (make them unique!)
Unfortunately, my favorite thing to do with MCPE is bug test, so I don't install any mods. You'll need some help from others to tell you which ones they find the most fun and why.