I used separate disks in order to partition my tables (the largest was 400GB). You will still get some gains even if your file groups are on the same disk, so I would not let that deter you.

The initial creation of the partitions can definitely take a while. I would recommend creating just one partition and doing it at the beginning of off-hours. Surround the SQL partitioning call with SELECT getdate() so you can get a good measurement of how long it takes to create one partition. That will help you plan for creating the additional partitions.