There are a few posts on SUN about website development. What you need depends upon what your goals are for your website, what you want and need it to do (functionally).
There is a great desire and tendency to want to do it yourself, and there are many programs and bundled website builders that can help you do it yourself. The important question is where is your time best spent? Learning to create, creating, and maintaining a website or devoting your time to growing, improving your business and providing customer service?
Like any of the business decisions you make, there are trade-offs, pros and cons to your decision. Less up-front cost for DIY versus potentially better results using a web designer.
At the very least, you might think about getting some quotes to create your site. Through the quote process you may discover what you need for your site even if you do not end up hiring a web designer.
Monstercommerce is a bundled ecommerce solution. A bundled solution has its pros and cons. Typically a bundled solution has some restrictions on design and certainly on moving your site to another web host.
I think the monstercommerce is a little expensive... the "pro" version is $999/yr.
Alternatively you could get a Zen Cart site hosted for around $90/yr, the solftware is free. Other costs a would be for a merchant account or PayPal to process credit cards.
If you wanted to change from the default Zen Cart look you could use a template - usually around $150 to $160.
Two things: I have created, used, and manage both osCommerce and Zen Cart ecommerce website... at this time I recommend using Zen Cart over osCommerce because it has more features included and is now more css based.
It may be easier to use a bundled solution (monster, yahoo stores, there are others too) that is what they are designed for, to enable people to start-up fast with minimal technical involvement. The drawback is living with some design limitations and your ability to move your site/online store should you want to at a later date.