Why don't you pursue the OPT route first? If you are successful, you could still marry her and possibly file after a significant amount of time has passed so it looks less suspicious.
To be honest - here's what I know - I know this guy who was on F1, did OPT and lost job and after exhausting all other options, he married a lady who is 17 years older than him, and has 3 kids. I have first hand proof (this guy doesn't know) that he married for the SOLE PURPOSE of getting LPR, I think even the lady knows about it. They don't even live in the same city. I was 100% sure the visa petition would be denied. To my surprise, he was approved after the first interview.
So, who knows.