if you are looking to use the truck to carry bicycles you will have plent of space, i can fit two entire bikes in the back of mine without taking any wheels off and still have room for gear, (i belive cargo area is 7ft)

engine sludge is always a maintance issue

mpgs will range depending on driving habits, environments ect, i get around 14 mpg in the winter, 17 in the summer (around town ) those numbers go up on the highway.

you can check the 4wd as described above but you dont need to go all the way to 4 lo (you should use 4 hi in most cases anyway) except for testing purposes