pcb design - Ethernet Interface PCB layout requirement (if any)

I am doing Ethernet communication to/from an MCU.

I got it working great with an ENC28J60 development module: https://www.digikey.com/product-detail/en/olimex-ltd/ENC28J60-H/1188-1044-ND/3471433

I recreated the module layout w/ DIP components on breadboard. It works pretty good though not as reliable, the obvious reason being that im using a birds nest of wires vs a module.

Now I am going recreate this interface circuit on a PCB. Are they any trace width/ length recommendation I should use?

I found this: http://ftp1.digi.com/support/documentation/022-0137_F.pdf

It says I should the keep the trace impedance below 50ohm for signal traces... But since Ethernet operates at different frequencies do i need to know ahead of time what frequency I'm transmitting at?

Should the Tx/Rx pairs be routed side by side or above/below on different sides of the PCB.

I am going to be using basic 1oz Copper, 2 layer board. I probably wont be able to keep the traces all on one side... Meaning the ground plane wont be flawless. How big of deal in this? I mean I did get it working on breadboard w/ jumper wire.

Thanks Tony