Well, how much hot water do you need at once?
I don't have a 'fancy brew sculpture'...I have one 7gal HLT aluminimum kettle, and one 9gal boil kettle. I have to put my strike water in my MLT (converted igloo cooler), then lift the MLT onto a desk in the garage so I can drain it into the boil kettle...then put it back on the ground so I can add the sparge water...and then lift it back up on the desk to collect the second runnings.
So, no, you do not "need" another 15.5gal keg for an HLT at this time...possibly never, depending on how you want your process to roll.
I don't mind a little workout on brew day, at least while I am still young enough to do that kind of lifting
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10