What career was Troy planning on?

I am fairly sure it was never mentioned that Troy had specific plans for his career after community college. My assumption is that he does not have anything planned yet. That is backed by several facts:

• His original plan was to play football and go to a elite university. That plan was ruined (by himself) when he dislocated both his shoulders before the big game, because he could not handle the pressure. He probably had no alternative career plan when this failed.
• His reasons to come to Greendale where rather arbitrary than a real career choice. From the community wiki:

  Ultimately, Troy chose Greendale because it was close to his house, and because the Greendale commercial had a student giving a thumbs up sign in a "cool way."

• As you said, he toyed around with several alternatives during his time in Greendale (plumber, AC repairman). If he really had his career planned out, he probably would not have picked those up that easily.

Apart from that, I think the dialogue with Jerry the Janitor from Episode 1x24 "English as a second language" also hints at the fact that he actually does not really know what reason he goes to college for:

Jerry: This is a life, kid. A real one. Doing something that matters. Something that makes sense.

Troy: The only thing that makes sense is this. Learning. Thoughts.
So I can think,
and get a student loan,
and grind my own coffee,
and understand HBO.

Jerry: You don't have to do that, kid. You're special. You could be a plumber!