I gotta argue with some of those figures:
A Cessna 150 is gonna burn closer to 6gph than 4. Good news is you can burn autogas is you buy the STC. It's a piece of paper that makes it legal, costs $100.
$20 an hour for maintanence? An annual where nothing is wrong would cost me about $500. I double that and budget $1000. Yeah, you could run into a major problem that cost more but setting aside $20 an hour for maintanence is overkill.
Engine reserve $8/hr? I've never worried about engine reserve. I wouldn't buy a plane I was planning on overhauling unless it was really cheap. I'd buy one with 500 hours on it and fly it to 1000 hours and sell it.
I own a 152 and haven't really worried about what I spend on it. One thing for sure, I don't fly it enough. I spend $15 an hour on gas (high grade autofuel). Set aside $10/hr for maintenance. Insurance is about 1K a year for full coverage. I paid 23.5K for it but dumped 10K into the panel, to include a Garmin 430 IFR approach approved moving map GPS/comm. I'd guess I might be able to get 30K out of the plane because it's an aerobat and the GPS. Check ebay, nice 150's can be had in the upper teens.