No, you'll need some stronger type of wood.
vult
Yes, just swap your bearings from skate wheels to longboard wheels, its all the same.
I got a sweet element cork longboard cruise off of craigslist for $50
4x8
Anything by original skateboards.
No, normal size for plywood is 4 x 8.
there is no best longboard for anything there just different but a good crusing board must be quiet smooth and easy to push.
purple orangutans are good
Ibn kelb
Look in the silverfish classifieds for a good fast longboard that is affordable and fits your requirements.
There seems to be good reviews for the Santa Cruz Stormtrooper longboard. Many who have purchased this and tried it out have said they really like it and it is better than other boards they have had.
there not the best, but they are pretty reliable