if you get the Lifeline GPL31-XT you will have 125 amp hours of capacity vs 100 for the group 27.
The 31T(105ah) or XT 125AH, is not all that much bigger than the 27.
A GPL-30HT is 150 amp hours and is taller. The more capacity you have the lesser the depth of discharge for the same energy consumed, and the happier the battery
Lifeline is top dog AGM, Make sure it can get at least 20 amps and preferable 40 when depleted to the 50% range.
Lesser $$ AGMS are good if one can't really meet the charging requirements
The Main cable from battery + to distribution block/fuse block/buss stud, should be fused, but Some will argue as long as it is heavy gauge wire/cable with solid wire terminaltions, and the total amps it is asked to pass never come close to its maximum ampacity, it can't possibly chafe on anything and short to ground, then the fuse is not needed. IE, Few cars that I know of have a fuse in the cable to the starter motor......
Generally the Fuse is to protect the wire, not the devices being powered on it.
Some people deliberately underfuse, and this is not really safer, and can lead to nuisance fuse blowing.
The fuse itself adds resistance, and nuisance blows might be difficult on whatever was asking for power when the fuse blows.
Wire Ampacity charts are helpful in figuring out which size fuse one needs to protect what gauge wire.
http://www.armstrongssupply.com/wire_chart.htm