G'day cracked pot,
I completed all seven subjects by correspondence, and I found that, out of the four "Hard" subjects, Navigation and Performance & Loading were the easiest to understand. So I would recommend these.
Both of the other subjects are achieveable by correspondence, if you put in the work required.
Most theory providers have a help line, which is handy if you get stuck. Also, there is always PPRUNE!
Good luck!
iamafish