Do you have the ChargeWizard to go with the 9100? If not, get one.
There is no need to match a converter to a battery. The charge currents are limited by converter voltage and wiring resistance so heating should not be a problem. All wet cell lead acid batteries charge with essentially the same voltages so you only need worry if using an AGM, maybe.
Use what will fit. Buy from a reputable retailer who offers a good warranty, sells a lot to folks who use batteries like you do, and will stand behind what he sells.
A group 27 is a bit bigger than a group 24 so it will provide a bit more energy storage. Figure about 10 to 15 usable (about half total capacity of the battery) watt hours per pound of battery. A group 27 is usually rated at a bit over 100 AH or 1200 watt hours. A group 24 usually runs about 80 AH or a bit under 1000 watt hours.
Keep in mind that things like temperature, cycle to cycle variance, age, and use profile can each vary available energy capacity by more than ten percent. What this means is that differences of that size don't mean much when it comes to choosing between batteries.
Watch out for those pushing hype. There is a lot of misleading information out there when it comes to batteries. If you can't measure it, don't give it much weight.