Yeah, but look at the conditions in Bangladesh - far more suited to spin bowling than NZ in general, and IIRC the pitches during that series were relatively flat but gave some assistance to the spinners. Shakib > Ashwin and Jadeja to begin with, let alone when the comparison is Shakib on a decent pitch for spin bowling compared to Ashwin and Jadeja on comparative greentops.
Ashwin is toothless away from home. No ability to beat the batsman in the air because of his action. He doesn't have the tricks of Narine either, and it's not like Narine has proven himself as a quality bowler in Tests on unhelpful surfaces (WW to rage). If he takes wickets, it will be through sheer weight of overs IMO.
Australia in Hobart were packing a Pattinson and a Starc. Given both operate between about 138 and 145 with their respective stock balls, and are both swing bowlers (Starc was getting nice shape into the right hander at 140 yesterday, ftr), I don't think you can say Australia was fielding a trio of medium pace plodders who didn't have the ability to generate pace or movement. Even Siddle can get it right up there when he wants to, and he was operating quicker in 2011 than he is now.
Saying Yadav and Best don't have raw pace is weird. It's legitimately what they get selected for - bowling 145km/h. Yadav has the better length and the ability to move the ball (though not necessarily on a consistent basis), plus is more intelligent. He's significantly better than Best, even if by no means a world-beater.
Zaheer's intelligent and experienced and shouldn't be written off. Even when unfit he was good enough to take wickets, and by the sounds of things he's in far better shape now. Even if he's not the bowler he once was, he's still good enough to take wickets. He's going to maul Rutherford, IMO.
No one really mauls Rutherford, mostly because Rutherford finds a constructive way to get himself out before any bowler can work him out.
As for your point on Narine - it kind of validates my point, you've got a bowler who has never done anything in test cricket, hasn't proven himself in test cricket however was the most trouble we faced against the Windies in the tests. He came out of that series (As he did the last time he played NZ in a test) with a restored reputation having taken a bagful of wickets against us for not many, despite being someone who hasn't shown the same ability against any other side in test cricket - which I'd also put in the Nathan Lyon category of taking wickets against us, at the point Lyon was at in his career, he was a "We don't have anyone else, why not him" selection - he's since shown a bit more quality but wouldn't call him world class.
Starc is quick, but slides the ball rather than gets bounce. When I think of pace bowlers who trouble NZ - it's either the relentless line and length bowlers, or bowlers capable of extracting bounce and movement off the wicket at a reasonable clip, someone like James Pattinson will always trouble us. Someone like Siddle will always be a pest for us, but we should handle a Starc style bowler relatively easily.
Yadav? I'd be surprised if he took wickets for less than 35. I think Shami will probably be in the 30-35 range, I think ultimately it will come down to Jadeja or Ashwin doing a Narine against us, because at the moment, our 3, 4 and 7 are as good as they've ever been and won't be troubled by any Indian pace bowler.
Go back to the West Indian series, which one bowler troubled us the most? An effective ODI player who had never really done anything at test level, except a performance he put in against New Zealand a few series previous - Sunil Narine wasn't even in the frame for selection initially, he played the third match and suddenly troubled us massively. We had no real issues with Tino Best, or Darren Sammy - we had issues with Sunil Narine.