The 6x7 format is described as the 'ideal format' with no cropping necessary for enlargements to common paper sizes; by comparison, a bit of material waste with the 6x6 format when enlarging. The 6x6 format certainly appeals to me but I only set my Zero Image pinhole camera to that format. Romance of any specific format aside, you use what is appropriate for the style and type of photography that should click naturally with you (not others!). Your smallish budget will I think severely limit the choices available to you: I would consider holding out to release more funds to build it up, maybe to double the amount. I am not a fan of heavily used ex-pro gear and would seriously consider looking at well-looked after, near-new or mint condition equipment.