First things first, what happens if you are denied credit? I'm sorry but credit is not near as easy to get today as it was three/four years ago. Have a job that pays $4000 a year and another job that pays $60 / month ($720 a year) for $4720 a year in income. Sorry, but no creditor will count donating plasma as income. Other thing, these jobs are considered seasonal employment, so the income is not steady and creditors frown upon that today.
You're gross income is $393 a month, and you think you can qualify for credit? Do you have any credit now? If you have student loans, those are on your credit report, and some lenders will count the payment against you. What's going to happen when you need cash and have to use the money from your job or plasma donation, then how are you going to make the computer payment?
Wait until you have the cash and pay for it outright. A lot can happen in 6 months, and the last thing you want to do is start destroying your credit in college.