Capacity planning for SQL Server consolidation
Like Randolph mentioned, disk is the kind of the last thing you need to worry about. If it's clustered as you mentioned, you can easily add storage to accommodate data growth.

The things that are harder to add and cost real money are:

CPUs (Licensing)
Memory (If you're on standard edition, you're limited to 128 GB)

In a consolidation scenario, you need to get a baseline of CPU use (easy perfmon--SQL Server CPU) and memory--harder (you'll want to track to page life expectancy as it relates to total memory use by SQL). Finally, you'll want to tack IOPS--the performance component of storage--if your source environments are on SQL 2014 or higher, there is a counter you can check (disk read/write IO/sec) to get that data. The disk data will simply be cumulative, whilst the CPU and memory data will need to evaluated. In a consolidated environment, you'll probably use a little less CPU and memory.

Finally, for a shared environment like this, you probably want to use Enterprise Edition with Resource Governor so you can prevent the "noisy neighbor" problem.