With the base material looking as dry as it does in the picture, after a wet Spring and Summer, I'd be inclined to skip the gravel base course myself. If this was in a situation where subsurface water was present, then gravel would be justified.
I've never been too keen on fiber reinforcement after experimenting with it on a few bridge deck overlays. However, using a 4000 PSI mix is definitely a step in the right direction, provided it's not poured too wet and has enough (grooved) control joints. Make sure the joints are at right angles to the walls and terminate at the support columns, because that's where cracking will want to originate from.
I'm curious what the "obvious reasons" are for you not wanting to make the pour yourself? You seem handy enough, and could save a ton of $$$. I don't know the dimensions of your basement, but judging from the picture, I'd guess the quote was up at $5000 or so, with the concrete costing only a tenth of that amount (based on 300 S.F. at 3.5" thick).