Notepad++ is the best freeware text editor, without a doubt. Additionally, you can download GrnEyedDvl's M2TW syntax highlighting language
here, which is a User-Defined Language for Notepad++. It doesn't work with the modelsdb, but it helps in other files. You have to check the "Don't use %APPDATA%" when installing Notepad++ if you want to be able to put it in the immediate directory, otherwise you have to put the files in your Application Data folder under your user. And the .API which was for AutoComplete I believe will not work any more, because the newer versions of N++ use .xml files for their AutoComplete.
There's a similar extension for UltraEdit32 by alpaca
here, but again, no .modelsdb coverage. Your best bet is just to run the python checker that KE made and gigantus updated to work with Kingdoms(shouldn't be hard to find), as it catches most errors. If your mod adds new factions, you will need to append the elseif faction list with their names, or they will be counted as an error whenever it parses them.