The $40 Win 8 Pro offer is good through the end of January. This also includes a free copy of
Media Center before Jan. 31st.
When you do download the ISO file, and get to the point, choose Install from Media you create. This will allow you to create a bootable DVD or Flash Drive from the ISO file. Having the DVD or Flash is very advantageous as it allows use of the Repair Console and allows for future installations should something go wrong.
I also always recommend a Custom Install. I realize this involves reinstalling all apps, but since your will be installing from a beta, this might be your only alternative.
If it is necessary (I am not sure if you will have to or not), using the Upgrade Media, even for a Custom Install, requires a qualifying OS to be replaced. Since you already have a beta OS, the installation may not see this as a qualifying OS. If this happens, you might have to use a Win 7 installation disk to install a basic Win 7 prior to the installation.