I recently found and then sold a Rectaflex Junior with Angénieux lens. The Junior is very similar to the 1300 except with lower top shutter speed. The body was not working but the lens was in good shape. I loved the overall build of the camera but the viewfinder was quite dim.
Yes, eBay is where I get most of the parts I need by buying junkers and pulling parts from them. It may take a while to get a reasonably priced one because sellers often overprice parts cameras.
The question is: Do all the ETR series models use the same mirror? If so, you can probably save by...
You might get $25-$50 for the Ricoh if it is in good working condition.
The other two used films that went extinct decades ago but some might pay $10-$20 for the Polaroid so as to convert it to modern 4x5 film size or to display. And, maybe $10 or so for the Spartus.
If the film wouldn't advance beyond 6 frames and you are using the correct film back and insert for 35mm then I don't think forcing it is a good idea. I would suggest finding out what is wrong and fixing that.