1 - the strenth in these side-grain joints doesn't need any reinforcement. The only reason to use dowels, biscuits, etc. here is to help keep the pieces lined up while clamping. Putting in too much of this sort of cross-grain stuff can actually weaken the joint.

2 - You can either use wider quartersawn lumber, which is more stable to begin with, or do as you suggest and use narrower strips. The trick there is to orient annual rings of adjacent strips roughly opposite each other, so one strip's tendency to cup in one direction is evened out by the adjacent strips' tendiencies to curl the other way.

3 - Having a sink in a wood top is pushing your luck no matter what wood you use. That said, curly maple is a fine choice.

I encourage you to research and think through the whole project before you start, particularly if economy is your reason for attempting it. Depending on the wood you choose you might be looking at $125-$200 for lumber, curly maple being even higher. That may sound like a deal, but if you have to buy a lot of tools and go through a long learning curve, you may feel you've passed on a $1300 bargain.

I'd use something really dense without open pores...that's why most butcher block is maple. You want to do something that's pretty much like the John Boos style, because that will hold up best.
The main tool you need in making something like this is a jointer, so you get absolutely flat surfaces for gluing. What you get from the sawmill won't have a flat enough or straight enough glue face, and it may warp if it isn't allowed to cure for a number of months. It's the quality of the glue faces, not physical reinforcement, that's most important. You also want the strips to be relatively narrow to reduce cupping. And you want to be careful how you affix the finished butcherblock to the island: if it's tightly screwed down, it restricts seasonal expansion and contraction and you can get splitting. (The usual solution for this is to have screw slots, rather than screw holes, on the cross-members that you attach the counter to, so the screw heads can move crosswise to the strips of wood. This is done with wooden tabletops all the time so that the top doesn't split.) You also want a lot of high quality bar clamps -- placed about every 6" -- to squeeze all the glue-covered strips; these should be on alternating sides of the two faces to keep things flat. Lastly, you'll want to see if you can find a woodworking shop with a large drum sander that can sand the whole thing flat once it's all done. I've done woodworking for over 40 years, and I'd think twice before undertaking all that work to make butcherblock. I will caution you that, if you don't have a lot of woodworking experience, high quality wood, and adequate tools, you could put in a lot of time and money and not be happy with the result.

One last thing...have you shopped around on the Internet for butcherblock? $1300 seems awfully high for something that size based on my past purchases.

I made a wooden counter top for a house I had in Florida made of cumaru, a Braizlian hardwood. It was two pieces glued together. I had a sink in it as well. I never had any problems whatsoever with the piece. I am going to make another one now for my house here in Brazil out of Andiroba, a mahoganny relative. The wood has been sitting around for 4 years and is very dry. This piece is going to be about 8 feet long and a little over 2 inches thick. Brazilian hardwoods are quite dense in general and I believe are a good option for this kind of application. Of course I live where they are abundant and relatively cheap.

You've gotten some good advice.

I made teak countertops for my kitchen. 30" deep made up from four planks.

Used biscuits and epoxy. Biscuits more for registration and alignment, but they do strengthen the joint a bit.

For wide planks, quartersawn is the best way to go. You may get cupping with flatsawn. Looking at the end grain of the board, in a quartersawn plank the grain will be vertical on the end. With flatsawn, the grain will be curved arcs, either smiles or frowns.

One concern with getting this wood from a sawmill. For wide planks, you might want them kiln dried. Again, that'll ensure that your countertop will not endure and drying cracks or stress.

The teak has pretty much been bulletproof. Looks great after several years, even with an undermount sink that gets daily use.

These tops were finished with mineral oil. They get re-oiled every four or five months, the area around the sink maybe every two months. I also made an end-grain butcher block from teak. It's about 4' square and 5" thick.