You have options (Score 2, Informative)

If you were to develop it yourself then yes you would save some money and spend a lot of time developing it. Other wise just outsource it! It costs you more but will be designed exactly to your needs.
I have not come accross any perfect out of the box solution for what you are asking. If you want to do it yourself i would strongly suggest after you decide on the platform being windows or unix look into getting someone to getting someone like verisign (more customers that way) for your credit card processing.
Then decide on the database for all your inventory. If it is already in sql or oracle or whatever you might be able to go with that and not pay a fortune but mostly spend time to design your own databae for all the inventory. Not to mention the ability to have a send to shipping solution (with other words no double checking) in inventory automatic deduction based on the online orders.
Don't forget if you spend more money now you most likely will save a lot more for the future! Look at the whole system not just the webstore front part.
The other important thing is of course make sure it gets advertised right otherwise you don't benefit.