Try it and see if you like it. (Congrats BTW)
I'm a shade over 6'1" tall, and 195 lbs... not as lean as I used to be, LOL. I ride a 163cm fairly stiff board.
I'd suspect that a 158cm board may be a tad small. I also believe that people are too concerned over centimeters, at least as far as snowboards are concerned. I'd guess it's probably better to err toward too short versus too long. I could probably ride my GF's 145... fat chance she could work my 163.
I'd say try it out. It's your personal taste that matters.
I haven't priced boards in a while, but I'd think that approaching the $300 range, there has to be some higher-end boards available, with sintered bases and such. Maybe a close-out from the 08/09 season? I typically prefer to find good deals on year-old new, closeout gear.
Ultimately, if you like it & it works for you, and you feel comfortable with it, then rock on.