Hello Groovybug,
for my personal experience:
1. Kid love to play and when I was 6 all my "not mounted" (how I called the raw LEGo material) was in a big uniqu box. Also find the right part is part of the fun!
2. Not sure on this, there also other places where to buy, personally I've never byed from pick a brick.
3. It depens from set to set. On how old is the set, the dimension, theme, but usally if you are not looking for a MISB you can find some good deal on the net.
4. For me it depends on you kid tastes, what he more likes, but as already suggested if he likes to mount and de-mount the set the 3in1 can be a good starting set
Hope I've give you something usefull :-)