I think it's up to you what you do. For the SCJD you have unlimited time, so you can buy it already or wait until you have did some reading and researching. Make sure if you buy it now, that you only buy the voucher to download the assignment and not the voucher to take the essay exam, because the vouchers must be used before certain time so you would have a time limitation (because the validity of such a voucher can't be extended)
The SCJD can be challenging depending on your background and goal. The assignment is really Swing and Socket/RMI. Everything else comes down to common sense and your interpretation of the requirements.
Just to correct Roel's post about the time limit, it has changed to 1 year for the assignment. I know this because I started mine just before Xmas 08 and I got 1-year limit.
I suggest do some reading on Swing and Socket/RMI. Do some code to test out how Swing events work. Learn about RMI.
When you buy the voucher, it depends on how much time you have. Doing the assignment during free time or as if it is a full time job is totally different depends on your background. Hope this helps.