Go
here for the external editors. I use ZZTAE, but KevEdit is just as good.
Also, if you're familiar with MegaZeux, you can make a character set in there and then convert it using Char Tools, which you can pick up
here. It's an in-game utility, meaning that you would load it like a normal game in MegaZeux.
custom char sets aren't very common in ZZT anymore, and I have to recommend against it, unless it would really make an impact on the quality or playability of the game.
As for toolkits, you can grab Dr. Dos', or if you want practically everything in the world, get my Ultra Toolkit, available
here (don't ask why it's in the Executables section). WARNING: Don't try to use the centipede heads or segments in the Ultra Toolkit, since they were created in ZZTAE which had (and still has) problems making them. What is technically going wrong is that thye're trying to follow the player instead of the head, which causes the game to lock up. Don't ask.
You can also download the Mystical Winds Encyclopedia v1.4 in that same Utilities section of Z2. The third file ($MWENC_C.ZZT) contains a set of toolkits that you might find useful.
Here is how to use a toolkit:
Load up the board that you want to use in editor. Press T and select Export. Type something and press Enter. Now load up a blank board in your game. Press T and select Import. Type the thing that you just exported. The board will now appear in your game.
Once you have the board in your game, move your cursor over something that you want to use. Press Enter. If it asks for things in the sidebar, just keep pressing Enter until it's gone. Now go to the board that you want to put the thing in. If the thing you grabbed hadn't appeared in the pattern box, it will now. Select it by pressing P, move your cursor to the place you want to put it, and press the spacebar.
If you don't want to bother copying the whole board over to your game, you can just grab something from one world, load up another, and place it. Transfering it over is good for saving time in the long run.
If you have ZZTAE or KevEdit, you don't need to do any of the stuff above. In ZZTAE, just press C to bring up a selection box with all 256 color combinations. In KevEdit, you can bring it up by pressing K.