I don't know a whole lot about ferrets, but I owned one many years ago and he did perfectly fine by himself. He had full run of the house, and I played with him every night. I also got one of those little harneshes and took him for nightly walks. I had a friend who owned a solo ferret as well, and he lived for many years happily. I think you'll be fine.
No clue on the flea stuff, but the cages...... I would look at garage sales, flea market and thrift stores even for a big one. Bigger is definitely better. My friend had a three story homemade cage, and the ferret loved it beyonds words. They really like hammocks and little tubes and stuff to play in. Have fun and good luck.