A good home network storage server holds all your files, backs up your devices, and runs for five to seven years if you build it right the first time. Skimping on drive bays or picking underpowered hardware means you will just end up replacing the whole thing in two years.
Mid-summer electronics sales are the sweet spot to buy, especially when drive prices drop. Manufacturers also tend to refresh their server lineups around this time, which means last year's model drops in price while still giving you all the power you actually need.
Remember that the intro window is a fixed deadline, not extra spending money. If you still have a balance when the 18 months end, the card's standard interest rate kicks in on whatever is left.