For the particular soapstone the contractor recommended, he says it would look best with its original color, not black after oiling. I was originally looking to get a soapstone for the black color, ie oiling would be ok. So now I'm wondering what I should do.

Here is a photo of the soapstone:

1) Modern aesthetic will not fit well in our period-influenced white kitchen
If you are going for a true period look, induction will not be authentic. But neither will a modern gas cooktop. There are ways to disguise an induction unit. The inspiration for my design was this from the workshop of David T. Smith.

2) Requires special cookware and dedicated circuit that will cost addl money
This is accurate, but hardly an issue. Any cooktop requires a dedicated circuit. Induction cookware is available at almost any price point. I started with a few pieces and add as I need. Some expensive, some very inexpensive. It gives me time to determine what works best with particular cooking methods.

3) Special cookware is very heavy and as we age it may become too burdensome
Depends on what kind of cookware is used. I have to admit, the cast iron cookware has been a blessing. I can lift more weight now than before I started using it. At some point, I may not be able to manage the cast iron. In my mid-50s I am not there yet.

4) May turn off potential buyers (if we sell for some reason) No one I know has this cooktop and only one of the GCs I spoke with knew what it was
GCs don't know everything! Only the appliance stores knew about induction technology when I was making that decision in 2008. Induction's advantages are becoming well known and prized as the price drops.

5) Can't char a pepper or use a grill/griddle on it
Everyone has their own cooking style, and some features are more important than others to each of us. There are ways to compensate for no flame induction. I roast peppers under the broiler. It is not as fast, but it works. A little torch would work as well. I use more than one grill/griddle at a time (on separate burners) quite frequently.

6) This is not the cooktop of the future but a 40+ year fad for gadgety folk
It may be a 40 year fad, but it will be replaced only when something better comes along. For now, it is the best we've got! The gadgety technology may be difficult for older people to master. I am sure my mid-80s parents and in laws could not get it right. If selling your house in future, you will likely have younger buyers who will demand the best technology. That is not gadgety or gimmicky, it is good sense not to prefer out-dated appliances.

Here are a couple pictures of induction in my period-inspired kitchen. HTH

Using a large cutting board over the cooktop for proofing bread under the lights. The house was cold!

The cooktop in context

I needed new cookware anyway, so not a big deal to pick up new pieces (I found a great deal on Circulon Infinite, it is also nonstick and DW safe, you can use metal utensils on it too). It's not heavy. The few pieces of copper I was sad to see go are now decorations on my wall:

As far as induction looking stupid or terrible or whatever in a traditional/old-fashioned kitchen, I disagree:

I think it looks fine, in fact with our counters it is barely noticeable and blends in quite well!