But having the hardware is vital. You have to test your design a lot. You're still going to need Vivado (which isn't cheap) and you'll need instance time to test the design on the real hardware with real workloads, along with any syntheiszable test benches you want to run on the hardware.
The pricing structure of the development AMI is going to be meaningful here, because it clearly includes some kind of Vivado license. It might not be as cheap as you expect, and you need to spend a lot of time with the synthesis tool to learn. The F1 machines themselves are certainly not going to be cheap at all.
If you want to learn FPGA development, you can get a board for less than $50 USD one-time cost and a fully open source toolchain for it -- check my sibling comments in this thread. Hell, if you really want, you can get a mid-range Xilinx Artix FPGA with a Vivado Design Edition voucher, and a board supporting all the features, for like $160, which is closer to what AWS is offering, and will still probably be quite cheap as a flat cost, if you're serious about learning what the tools can offer: http://store.digilentinc.com/basys-3-artix-7-fpga-trainer-bo... -- it supports almost all of the same basic device/Vivado features as the Virtex UltraScale, so "upgrading" to the Real Deal should be fine, once you're comfortable with the tools.
