Re: What battery should I buy?
Do you have a charger that can charge anything other than wet cells? Even if it's a smart charger and does different battery types many can only be configured for only one type of battery type at a time so you can't mix wet cell, agm, or gel. Similar issue with alternator charging.
I'm cheap and like wet cell but they need a certain amount of maintenance. Other people I know use agm. I don't currently know anyone using gel.
I have a 37hp diesel that starts up just fine with storage batteries. I have read that as engines get bigger and need more current for starting there is a point when a starter battery is preferable for starting.
There is a lot of material out there. Start reading and pick your poison. :-)