Upgrading to 5/05 is the cleanest and easiest option. The Digi One IAP does support two serial ports so you can bridge to Ethernet and keep your local HMI on the same serial port. Depending on what you are doing on Ethernet this might bottleneck the serial port, but for just occasional access with RSLogix, you probably won't notice a problem. It has been a few years since I set up a Digi to use both serial ports, but IIRC, I could upload and download to both the Panelview Standard and the 5/03 over the Ethernet bridge.
And to clarify, yes, you would need a Digi for each machine.