Pine will work just fine. It's not really the kind of wood, as it is the size of the lumber, the method of construction, and freestanding-vs-supported against a wall.
Basically, you need to build it sturdy, Most any type of wood can be used.

My soapstone and marble sink was made by Florida Joshua, so I asked this question of him when I bought the sink.

I used an over-the-fridge cabinet as the base for my sink- it was 1/2 inch plywood sides, and he recommended I use a 1x2 to reinforce each corner and screw those into 2x4's set across the top of the cabinet, as well as 2x4's actually all around the top of the cabinet, which is what the sink sits on. All the wood I used was fir, and I just painted it to match the inside of the cabinet, so it's not really noticeable.

My sink is 18x30x9. All wood was screwed together with 3 inch wood screws and watertight glue. My cabinet is free standing, actually the exact size of the sink and does not butt up against the wall. The sink does sit proud of the cabinet by 2 inches as well, but is also held down by the soapstone counters.

HTH!

You'll basically be constructing an extra sink support 'cradle' out of sturdy 2x4's under the sink in addition to the actual cabinet, so I don't think the choice of cabinet wood is too essential in terms of structural strength.

Dando, thanks for your information on sizing of the wood being the determination of strength for the most part. The other bit about freestanding vs up against the wall will be important for me. The sink will be up against the wall but I still want it to be able to stand on its own. I haven't decided on the style of base. If it is open at the bottom and standing on legs, I will be tiling behind the sink. So the outfit might be slightly out from the wall. I still want to anchor it to the wall.

Oldhousegal, Great explanation of how you did your sink. That is the exterior front to back depth I plan on doing too. Did you go around the top of your base with 2x4's and than on either side of the base another layer another of 2x4's on top of this? I'm just a little unclear about the placement of the different 2x4's you've listed. The setup sounds very strong.

Circuspeanut, Thank you for your "cradle" analogy, very helpful and a good visual.

I will be sure to share this information with my carpenter. We haven't talked about the specifics of construct yet. Since this is a little out of the ordinary, because of the heavy sink, I am glad to have this info from GW to help me when we do talk details.

my showroom has a soapstone farm sink in it. the post is correct you support the actual stone sink with wood supports under it inside the cabinet. The floor of the sink cabinet needs to be strong enough to hold the supports. 1/2 plywood should be fine

Here's a better explanation of what I did- sorry for the confusion!

I cut a piece of plywood, the same size as the cabinet, and set it on top of the cabinet walls, which acted as the base for the 2x4's.
I screwed the plywood to the cabinet sides, then screwed 1x2 fir members in every inside corner of the cabinet- and these were attached to both the plywood and the cabinet top.
The 2x4's were screwed together to create a rectangle that fit completely around the top of the cabinet. and then screwed to the plywood.
I "hid" the 2x4's with a piece of molding, and eventually put in a drip rail of a soapstone shelf topped with marble tiles, so you'd never know it was sitting on 2x4's.
I was told this was a bit of overkill (and I did use a lot of screws!), but I figured I only had one shot of this and did not want to risk hurting my sink!

I used a hole saw to cut the opening in the cabinet top and plywood that was 6 inches wide (enough room for hands and the drain). Because my sink did not reach to the wall, I had to cut a significant sized hole in the back of the cabinet for plumbing access.

I hope that gives you a better mental picture of how I did it.

Oldhousegal, Thanks for your explanation, I think with the info I am going to have a good picture of the needs. I will keep everyone posted in the next few months.