Help choosing an FPGA

A SoC (system on chip) contains both an FPGA and one or more processors on the same chip. This does not mean you need to use the processor, you can leave it unconnected to your hardware, the only thing you need to check is where the peripheralsare connected to. If, for example, you want to use leds, it might be that they are connected to the processor and not to the FPGA part of the chip. I have no experience with Altera SoCs but there might be some trickery you can do to control the CPU peripherals from the FPGA part. But why not create your design first? then you get an estimate of the resources needed, and you can select the chip you need based on those numbers.