From my experience as well as many a forumite:
You WILL want to upgrade later. If you can swing it, get a 40+ qt pot and an outdoor propane burner right off the bat. No reason you can't use it for the smallest of batches, and it will allow you to do 5- or 6-gallon batches (AllGrain or Extract) when you want to.
However, if money is tight, a 5 gallon pot is fine for partial boils. You can just top off with store-bought filtered water, boiled and cooled water, or (GASP!) even tapwater with little risk of trouble.
You'll just have to balance current funding versus future flexibility and capacity. I always advise: the more bigger the more better.