I have not owned a Nero 3, but have a Nero 5 that I used for around a year on a 100g and then broke that tank down, its in storage now, but it worked great for the time I used it. I’m now using a Jebao SLW 10 which is (I think 1096gph) on my 28g, and I kind of wish I would’ve gone with the 20. In my tank, the 10 is adequate for a mixed reef, but it’s just about maxed out, I would need to add a second or upgrade to a larger pump if I was doing an sps dominant tank. The thing about both the Nero and the SLW is that the flow is very wide and broad and less direct, so even at a higher gph, it’s less likely to blow your sand around, and since they’re adjustable, you can always turn it down if it is. For your tank, I would go with either the Nero 3 or SLW 20 (maybe even the 30), or around 2000gph.
As far as the warranty, in the US at least, I think you’re going to have a hard time finding anything over a year. I think Amazon might offer an extended warranty, but I’ve always recommended against extended warranties, they’re a cash grab and every consumer protection agency recommends against them. So long as you clean it monthly, they should last for years.
