Hi Meniscus.
I can totally understand your situation.
I am someone who seeks knowledge over money but again, who am I kidding. I want money as well for what I do. I don't mind doing things I dislike or give no value addition if it would help me get to reach a bigger and better target.
Coming to your situation,
Having a PhD will undoubtedly bring a great value to your CV and who'd say no to a "Dr." prefixed before their name.
I'd suggest you go for a PhD only and I repeat only if you love your job cause once you're done with those three years, then it's the rest of your life researching and mastering it.

But again, an engineering degree coupled with a masters degree in the field is good enough to get you the money, while doing things you love.

A Final word,
I'd say don't go for a PhD as yet, but aim to master in your field and take in as much experience as you'd like. Earn the money you'll need and then you could take a break to pursue a PhD later (in your 30's). It's not going anywhere as you can do it whenever you want.

Hope it helps. All the best!