I used the cage in the link you posted for a foster ferret of mine (she came with it). I was able to rearrange the cage (and take out that stupid slide!) to make the floor levels more balanced and would save her from falling. But I SO do not reccommed that cage. Ever. It was a pain in the *** to clean, and rearranging those shelves? Took me Two. Freaking. Hours. I loathed that cage.
And really, I don't even see the point of ramps/levels if they aren't large enough and secure enough for a bed. If you can't afford a FN 142, then I would personally get a FN 141. You can always add another level later on.
Ferret Nation Habitat Model 141 - Ferret.com
I think my two could comfortabley live in just one of the levels of my FN142 cage. The only thing I have on the bottom level is their litterboxes. Everything else is on the top half. You could even by just the cage and forgo the stand and only spend $100 (although there would be a hole on the floor to allow access to a lower level.
Ferret Nation Habitat Add-On Model 143 - Ferret.com
I <3 my ferret nation. I had another Super Pet cage (four full floors) and it was such a pain and was so cheap compared to the FN, even though the two cages retailed for the same price. (I got my Super Pet cage free, or I wouldn't have gotten it.)