What Bruce said. If they like the film as long as it looks OK and they actually want it they will buy it. Even the expensive new star trek series is 1080p only on UK Netflix
In addition to 720p stuff on Netflix theres quite a lot of standard def Digi-Beta and Beta-SP. Plenty of very rough looking old BBC sitcoms.
They made a big deal of acquiring the monty python catalogue, most of that is a mash up of 1"C format and 16mm. Format isn't barrier to purchases. There's usually a minimum technical quality they will accept - but thats not just in terms of resolution (since they will take SD). Sound quality, chroma and Luma levels, blanking etc.... I used to work in QC for a UK broadcaster/film distributor and have rejected many many television programmes and films for technical reason's. 90% it wasn't because of a substandard camera format - but usually it was due to audio problems and mistakes made in the edit. The choice of camera is far less important then your choice of sound recordist and post production team in getting a film/show past QC.
If you have a C300 and the film looks good - it won't be the lack of 4k that prevents a Netflix sale, more likely lack of stars and the plain of numbers game of it being a real long shot. Blue Ruin (shot on C300) was on Netflix
4k is only needed for a commission, at that point if they want 4k, they will give you enough money to shoot 4k and tell you what cameras are allowed.
As others have said use the best camera you can afford for your project. If you already have a C300 then shoot on that. But if your hiring in maybe check out the Ursa mini and Sony FS7 as well. They rent for similar amounts and may give you a bit more latitude for colour correction. I'm a bit fan of the C300 - but the 8 bit files do limit your flexibility in post. Stretching to FS7 or C300 Mk2 might make your life easier on the shoot - regardless of the Netflix rules.