When cooking in a slow cooker, you do not have to precook the leeks. I made a potato leek soup not too long ago that called for throwing everything in the pot and it came out great. The only thing I cooked before hand was the bacon, rendering out much of the fat and giving the nice golden brown and crispy bits to sprinkle on top.
You CAN precook anything to help reduce the cook time and I always sear meat, but unless you are going to caramelize the leeks, you will not notice any difference in flavor or texture.
As for the pureeing part, you want to be careful pureeing potatoes as they can turn into a nasty gluey mess. Just keep it to a minimum or use a food mill and you should be fine. I typically just use a potato masher and leave my soup a little on the chunky side.