Here is what makes me uncomfortable with what you want to do. You say you want to buy a computer you can upgrade. Most off the shelf/out of the box computers have warranties that are voided if you open the case sooner then 1-year. You may have to have it done by a pro to keep warranty in place and that may cost more for less computer in the long run.

Brand new computers, fresh out of the box have been known not to work. I have had to return 2 brand new computers before I got one that would do the job. With your timetable I say WAIT if you can.