@AuraObscura - It seems that you have thought this out clearly and that you have a solid reason for getting into a PhD program. Yes there are sacrifices to be made but as long as you go into it with your eyes open, your age should not be a major impediment. The fact that you already have an MS may let you avoid a couple of years of coursework if you choose a program that facilitates students coming in with work experience and completed coursework. You could very well complete the PhD in less than 4 years. The key is motivation and is sounds like you have it.