If you have the headroom, also consider the Trojan T125 or T145. These have the same footprint as the GC2 (T105 in Trojan line.) batteries, but will give you some added reserve capacity in AH's.
Yes to a water fill system.
And yes, AGM's are also available in GC2 sizes. They do charge a bit faster, and are very low maintenance as far as say a yearly cleaning of the posts (Though quite often that is not even needed, but good to do, just to ensure good contact.). Fullriver has been getting good reviews for a lower price AGM. We run Lifeline, and they will probably be on the top end of your price point. All of this, is assuming your charger has AGM settings available.
Best of luck to you,
Smitty