I have made several Tekkit classic plugins (for the server), use the tekkit.jar server file as your api, that way you have access to the craftbukkit classes and the underlying minecraft ones too
as for accessing tekkit materials, i found its better to use the typeid's rather than the type ,
here are a few little tricks when working with plugins for tekkit servers
How to get the tileentity of a block
CraftWorld world=(CraftWorld)block.getWorld();
tileEntity=world.getTileEntityAt(block.getX(),block.getY(),block.getZ());
if (tileEntity instanceof TileEnderChest){
//this is a enderchest
}
how to change bukkit world into minecraft world or worldserver
CraftWorld cw=(CraftWorld) world;
WorldServer ws=cw.getHandle();