I like the idea of a "forward deployed" temporary base. It would have great atmosphere: spartan, unfinished, uncomfortable.
I don't like the idea of a new ship class as much. Why would Starfleet build that class instead of Defiants? In my campaign I'm going to slip Defiant's engines down one step and replace the pulse-phasers with equivelent standard phasers. That solves the over-powered engine problem. If you're going to have a new ship you need to explain why Starfleet went to the trouble.
Same with the NX-series Intrepid. Why is it assigned as the "big" ship here, when it isn't working at 100%?
You can do these things, just come up with a reason.
“I am a soldier. I fight where I am told, and I win where I fight.”
General George S. Patton, Jr.