I had quite a few multi hours before I went to the Aerostar, but no IR. The insurance was cheaper than on the older "simpler" plane! They wanted me to do type training with an approved instructor, but otherwise no minimum requirements of hours on type. So in my experience, no, it won't cost you more to insure. But if you have very little multi time, I'm sure they'll bump it up a bit for the first year or two and probably ask for 10-25hrs in type with instructor. On my first multi, they wanted 15hrs in type.
But here's a really powerful tool - if you give up hull insurance at the start, almost no one will deny you coverage. Plus, it will be much cheaper. You can always add hull later if you feel you need it. My reasoning is: if I ding it it's probably minor and I'll pay to fix it. If someone else dings it, they'll pay to fix it. If for some reason I ding it so bad it can't be repaired, then it's likely I probably killed myself anyway.
As for what plane is best, you have to come to that conclusion yourself. All I can say is that bang for the buck the Aerostar is hard to beat. A P Baron is slower on the same fuel burn and has that 10000hr limit, plus Beech prices for spares. The cabin is also narrower in parts compared to Aerostar and they're more expensive to buy. The visibility is worse and they're not built as sturdy. I don't know much about the 421. Certainly the cabin is bigger.
As for the wisdom in going to a twin with few hours? Well, I did it and so did a few others here, but it' certainly wouldn't hurt to build some experience in a simpler plane. But, should you chose to jump in at the deep end, it's not harder to learn to fly a complex machine, really. It might take you a few hours more, but once you've learned, you've learned. Learning to fly a taildragger well is probably more complex than learning to fly an Aerostar once you know the systems.
All flying is good flying!
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