I'd try really hard to avoid implementing address verification. Unless i worked at UPS or FedEx.

I'd try really hard to avoid implementing spellcheck. Unless i worked on Word.

If it's a core to survival thing, yeah, you should probably roll your own. If you can't beat OSS state of the art, well, you've got a problem.

The other heuristic i think is generally good, go for a couple of big dependencies over a whole bunch of small dependencies. Coping with the interaction between 3 huge libraries is so much easier than 100 tiny libraries.