Convey interest in taking graduate classes in fall with keeping in mind possible PhD in future?

[Texan_Air]

Some background, is that I finished my aerospace masters this fall and I have a bachelor degree in mechanical engineering. So I am going to be starting a government job as a aerospace engineer and be placed in a 1.5 yr rotation program with different engineering groups on aircrafts. My gf is going to be going to University of Maryland at College Park, and I want to start taking classes there as well next fall semester (loving the idea of graduate CFD classes and rotorcraft aerodynamic classes). In the future I would love to do a PhD, but I want to wait till I finish my rotation program. After that I would want to use a complicated project from my work to put as a PhD project. However, I get confused on how to convey that I want to do graduate classes in the fall to UMD  with the hope of pursuing a distant PhD in the future on the application process. The advice I get from my masters advisor is to apply as a PhD and not a masters student, and that I have a great shot of being selected because my job is one that pays for PhD education while working (so no funding needed as much).

I'm thinking of conveying this in the SOP, but I get also confused on how to contact a PhD advisor for a research area that I am not sure about (because that can change slightly depending on where I end up in work).