The only way to forecast space needed for a database is to trend and bench mark growth of the existing database. That or use a crystal ball. Even forecasting can be very wrong if there is some other factor that would warrant drastic growth. The simplest way to track this is to create/use a utility database and write the file size of your mdf, ndf's, and ldf's to a table. Make sure to include a date time stamp. Then over time you can calculate how much space your database is growing week over week and month over month.
To answer your question on what type of disk to give the optimum IOP's, well again, what kind of IOP's do you need? If you have a static database that is not heavily read/written, then you could get by with slower and fewer disk. If you have a high transaction database you need fast disk and lots of them (the more spindles the more IOP's) With the higher density disk, getting lots of IOP's is becoming much harder without wasting available storage. If you need lots of IOP's then solid state could be the answer if you have plenty of budget.
I hope this helps, I could write a chapter on this topic.
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