I've assembled a few laminate countertops over the years. You need some tools you may not have including a router to cut the laminate edges after assembly. I recommend doing a wood edge rather than putting laminate on the vertical edges. It looks better and holds up better. I've done this and used a rounding over bit on my router to shape the wood edge and cut the laminate. If the top overall is less than 5' wide by 12' long you can buy laminate sheets at the home center in 5x12 sheets and eliminate the need for any seams. This is a big help. If you do make your own, definitely use plywood for the base rather than particle board. The cost difference isn't big and the plywood will hold up better and is easier on your saw blades than particle board.
With the sink in the corner, I would steer away from stock post-form counters because the seam will go right through the sink and will be a potential problem area down the road as water gets into the seam and delaminates the particle board substrate. If I did use post form with the seam at the sink I would definitely seal the seam before assembly. Paint it liberally with an oil-based polyurethane on the edge at the seam and around the sink and several inches on the bottom at the seam and around the sink.