its not overkill, but a cylinder that large you'll want to store outside the kegerator/keezer, which means you'll have to drill holes and avoid hitting refridgerant coils or you toast the kegerator.
a 5lb cylinder should carb and dispense 8-10 cornies. I don't like priming cornies because you create additional yeast sediment. To each their own
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