Build an 8-digit display using Seven-segment Display and 74HC138
Using an external decoder for digit selection is a good idea, since it will save on Arduino pins (but you'll still need 3 address pins). And you are correct that you'll need boost transistors. Connecting a decoder with active-low outputs, like a 74HC138 to transistors to provide active-low display drive is not exactly simple for beginners, since you'll need 2 transistors per channel. You can do it like this







simulate this circuit &ndash; Schematic created using CircuitLab
You'll notice that it takes 2 layers of function. Since you're already buying a decoder, you can save some circuitry if you instead use the 74HC238. This is identical to the 138 except that the outputs are active high, and you can do this







simulate this circuit
instead. Using the components shown, you'll have no trouble drawing 50 mA per cathode, and 100 mA is entirely reasonable.