You will get tartness and a little flavor out of the fruit but not a lot of sweetness. You will want to freeze the berries first to burst the cell walls. When I've used raspberries, I mashed them and heated them to 160 F to sterilize...this may or may not be necessary, but I was using home-picked berries and wanted to minimize any wild yeast.
If you want a touch of flavor, 2 or 3 lbs is fine. If you want to really sock it to the beer, 5 or 6 lbs. I assume this is a 5-gal batch.
(2 qts, equal to ~4 lbs, of blueberries in a 3 gallon batch added a very noticeable aroma and tartness to a batch I tried once).
I would probably add them late in the primary, like you suggested, to clean the beer up (lots of seeds in raspberries). The yeast will eat up virtually all the sugar. Are you using a secondary?
Regardless of how you clear it, there will be enough yeast in suspension to prime the beer.