DIY: you have all your business info under control. Each modification project could cost you from $50 - $300. You could have freelance programmer make the change for you. The Out of box shopping cart software is good enough for you to run an online shopping site. You need to lean how to maintain the site or that will cost you more because it cost a lot of time to load all your products....
Since you are still in planning/testing stage, you could try hosting service like Magento.go. $20/month could start. I never use it but it seems pretty good in functions. Magento has DIY free version too.
