By text expander, do you mean you want to type a few characters and a software to replace them with predefined text? Maybe the easiest (also free) way of doing that is using AutoHotKey. It can do that, and a lot more and it's fairly easy to learn even for non-programmers (like myself). The pkl is actually autohotkey script.

If you're looking for something that will check your spelling and will autocomplete the words for you though (like on smart phones' keyboards), I don't know of anyone who has done a complete script for that with AHK. I'm running a universal AHK spellchecker on Windows but I don't know if that is what you're looking for.

Thing about using PKL though - if you run two keyboard hooks simultaneously with AutoHotKey they'll compete. So if you want to use completion you should do it by including your stuff into the PKL source then recompiling.

There are several autocompleters already written using AHK. Search the AHK forums for instance.

If you want the autocomplete to be fully automatic I don't see a way around this. If all you want is some template strings I suggest using PKL's Extend functionality instead (e.g., pressing Caps+h will write out your standard header).