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I ended up doing xcart which has not been very friendly for changing, but a whole lot better than starting from scratch.
What you need to ask your self is how much more can I make by getting my products online ASAP? In most cases you will make that $500 back in no time. Then when you have money comeing in you can look at designing something from scratch.
If this is for some client that is willing to pay you $10,000 then go for it. Recreate the wheel. Either way you have to drop another $1000-$1500 on SSL and CCG.