Sheetrock is the least expensive way to go. For a drop ceiling, you need at least 3" for grid to be able to tie the wires and get the tile in. And it ill still look "commercial". And have you priced ceiling grid?? Not Cheap. The Armstrong 12"x12" stuff requires additional furring, you'll need some trim around the edge, and the CHEAPEST tile they show on their site is $1 a foot. You'll probably want to upgrade.
Use some of the money saved to rent a Porter Cable drywall sander with the vacuum to do your sanding.
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