Funny, I'm in a very similar situation. I'm in college, renting an apartment, and I did buy the exact same filter. Money was tight for me as well, so I didn't get the extra tank. I don't really think you need it. The GPD rating on that filter is pretty high compared to other ones, so waiting for purified water shouldn't be too much of a problem. I'd say that I lose 2-3 gallons a day due to evaporation and it only takes me 10-15 mins. to refill.
But what you should do is invest in longer water tubing for the exit portion of the filter. I have my tank in my room upstairs and have the filter installed under my bathroom sink. This way I just run the long (about 20 or so feet) tube straight to my tank. I don't have to carry any buckets to and from the water source.
Also, if you're moving soon, I wouln't recommend drilling a hole in the drain under your sink like the directions say to. All I do is use the supplied (black) waste water exit tube and run into the sink right above it. That way it goes down the drain like it is supposed to and you don't have to drill anything.
Buy this filter!! It's a steal for what it provides. Your tank will love you for it and you will always have drinking water ready to drink! HTH