I'm pretty hard up student with a part time job as well.
I'm looking at getting the i5 13" in a few days/weeks.
I was thinking about finance. But decided I'll go for the buy now pay in 6 months option. I can easily earn enough to put away over 6 months.
Though the 15" would mean about £220 a month (x6) with university discount, so it depends on whether or not you can put that aside.
On apple's site it says you must have a full time job, be over 18 and have a fixed address. I'm not sure about student, but i checked in at an apple reseller today to see what my options were and he didn't say that I couldn't.
Anyway, back to my point, I'm going to save an pay the lump in 6 months, instead of worrying about the money definitely being in my account for each direct debit.
Ross.