Although I don't have extensive window experience, I do have enough to point you in the right direction.
You should not have to break any brick to get the window out, unless there is brick that was built around it after it was installed.
You can either use cement screws into the brick like you did with the bottom plates of your basement walls, or you could build a wooden frame first out of pressure treated wood.
When you measure it, you always take the smallest of the measurements. In other words if you are measuring for the height: first you measure the height of the left side, then the height of the right side. If the left side measurement was 23 7/8" and the right was 24", you would use the 23 7/8" for the height. Same for length. Then order it that exact size. After you install use expanding foam around the window gaps (that stuff is pretty strong when it dries and will help hold the window in place). Cut the excess expanding foam away and install your mouldings.