As far as I know, Premiere Elements has always created .PREL project files
I think what you have is a project file created with Premiere PRO, a completely different product
Then what are my options to ensure I don't lose months of work? Should I return 13 and try to find Pro?
There is no Premiere Elements 6, Premiere Elements (any version) project files have a file extension of .prel and not prpoj.
There is a high probability that you were working with Premiere Pro CS6, a different program than Premiere Elements (any version) and very much more expensive than Premiere Elements.
If you purchased Premiere Elements 13 as a download from Adobe and you are within 30 days of purchase, you can request a return/refund from Adobe Orders. If you purchased elsewhere, you are governed by the return/refund policy of the vendor who sold you the product.
The prpoj file cannot be opened in Premiere Elements (any version). You could consider exporting your prpoj from Premiere Pro CS6 (if that is your program) as a .mp4 file and then importing the .mp4 into Premiere Elements. But you will be dealing with a whole video which will make significant further edits in Premiere Elements difficult. But if you do not have access to Premiere Pro CS6 or whatever Pro version you had, you are not going to be able to export your .prpoj file.
The one perk in all this is that the Premiere CC is now marketed only on a subscription basis which might fit into your budget better.
But, see if you can verify what program gave you the prpoj file. More details you have, less likely to run into surprises and setbacks in your plans.