Don't do it. The most important part of a PhD is "spending years surrounded by smart people doing research in your field" -- and part-time students almost completely miss out on that.

I have plenty of interactions with some of the best in my field; I work as an engineer with some guys who have done groundbreaking work. However, I have personal reasons why I wish to complete my education with a doctorate. Advancement at work is definitely not one of them as I'm doing quite fine without.

Since I'm doing the work myself in the evenings, it would be nice to get some paperwork along the way.