At least you do have cut offs. in case of an overflow, you have to go downstairs to turn off the water. if in the bathroom, you will save on some distruction.
Its your call and your budget requirements.
As far as the drain cutoff. This can easily be done and not that big of a deal. Just don't cut the drain going into the wall too short. You want a good three to four inches protruding from the wall. When cutting this pipe, cut it squarely and without crimping or denting the pipe. This will cause you to cut off more.
Take your cut off piece to the shop and have the proper connections made for the link up. Since the cabinet will be gone, the connections will show, but you could use a radiator hose type clamp to make the connection, but it looks unsightly without a cabinet.
That is good that you have the flex connectors. Nice and easy to work with.