Do you have spotlights that really light stuff up at 300 yds? I ask because if you're talking about hitting something from long range under just moonlight, the requirements are going to be a lot tougher than if something is lit up well with a spotlight.
Though I've mostly done this at closer ranges, I find if something's lit up with a light many scopes can work well. Size of objective, glass quality, etc, doesn't matter all that much. Having a nice thick reticle and good illumination help. But it's not a difficult task, you can do well with a much cheaper scope.
Going without a light, especially at longer ranges, the scope needs to be able to see the target first. This is where the high quality expensive glass really comes in, as well as 50/56mm objectives. The illumination system needs to be good as well, having settings low enough it doesn't wash out the target. This is where you can spend some big bucks.
Though as you said, you can quickly get into spending some silly money for killing foxes. Before dropping the big money I'd be inclined to try out a 3-12X56 Meopta with the illuminated dot 4a. Illuminated dot 4a's simply rock for this sort of use, from a dim glowing dot in the dark to a brighter glowing dot with a thick, quick to see reticle surrounding it in low light or under the spotlight. 56mm of Meopta glass will still see the target after many much more expensive scopes have gone dark. Anyway, that's just another suggestion you may not have thought of that could save you some money.
If mainly spotlighting at closer range, you could probably get by pretty well with an even cheaper scope. But then again, if you can afford a couple grand...a high end Zeiss, S&B, Swaro, etc, can bring you much joy.