FWIW I have been running my 60Hp with a built in VHF on a 60Ah bog standard car battery for the last 5 years. (it was as a result of the original battery that came with the boat keeling over mid me being a rescue boat at a week long event, and was all the local Kwik fit had to hand that would fit in the console). It has since died by toasting, as I hadn't realised the Merc Clamshell rectifier is just that, so it had seen the best part of 50+ hours being "toasted" at upwards of 19V! I have since replaced the rectifier with a regulated one off a honda.
The toasting reduced it to the capacity of a couple of AAs, so I replaced it. Got a deal on an 80AH leisure batt, as I now have a plotter to feed as well..... F*ing ell it's heavy, especially compared to the same sized one that came out!!
Some on here would probably reckon 80Ah isn't enough, but I have a Bulgin Buccaneer on the side of the console - it sits at home on a trickle charge, and gets a top- up from the car's "caravan" socket en route to the slip.
Once your engine has started, (and being a 30HP you can probably pull start it pretty easily) then it's doing next to nothing. Even if it doesn't have the guts to crank, you can walk to the transom & pull.
If kept well, a small car or big motorbike battery should do the trick nicely, even if you accidentally abuse it for 5 years!