For a long range escort, you need a long range fighter. None of these are actual dogfighters, so as marcsand2 notes, you need a plan for when things go badly wrong.
Personally I think the guns on the Vanguard are about the best you'll find for a sniping machine. It carries the only S5 gun mount in a fighter. You need to practice shooting at distance where you're lack of maneuverability is not as important. The turret isn't great at distance but I am hoping it will work well for anti-missile duty. If we can shoot down incoming with a pair of Jokers or some such, that makes this an amazing distance fighter.
The Vanguards have the fastest missile lock system in game, and very few fighters carry S3 missiles. If you want to out-think opponents, and perhaps even cause them to avoid what you're escorting, a Sentinel might accomplish that for you, and you waste no ordinance. If you want to punch hard, the Harbinger can be loaded with all S3 missiles, and there is nothing that moves as fast and can carry that missile loadout.
So yeah, if you want to fly long range escort for exploration, I doubt you can do better than the Sentinel or the Harbinger. Just you need to practice fighting styles that play to the strengths of the big gun, especially range. Learn to fly backward.
View: https://www.youtube.com/watch?v=IRXTqMj4TC4
