We did a sea trial and survey Monday on a Formula 34PC with 7.4 MPIs in it. They wouldn’t turn over 3100RPMs and then we found the entire engine room bulkheads and stringers would peg the moisture meters. That deal was toast. We are going to look at a 36PC tomorrow. It has 502s in it. I hope it is in better condition.
I had the 31PC Formula and it's really a quality pocket cruiser. Comfortably cruised at 30 kts at one mile per gallon and would top at 40.
BUT the damn thing has Volvo gassers and the reason I got rid of it. There's a few of the Formulas with diesel, which would be better, but most are Volvos... so probably puts the Formula out of the question.
Having had inboards, outboards, I/Os, diesel, gas, singles and twins there's something I'll NEVER HAVE AGAIN... a gasser I/O. It will either be a diesel or outboards.
The outboards have come a LONG way and are very reliable, easy to maintain and efficient. Also, for the pocket cruisers (mid 30s) most of the OEMs are going outboard.
You could stay with a diesel... get a really good reliable one and hire your gas mechanic to change the required stuff...... oil, filters, belts, etc.... easy peasy. And perhaps have a periodic run to a diesel expert for a look see. They just last.
Now, how much cabin room do you need? Just you and the wife or will the GF come with, too? Overnites, galley, AC, etc? or just day trips?
And if you're in the mid 30s, most any boat will cruise about 1 mpg or less.