If keeping costs down is vital, perhaps you should consider a two stage approach to getting the business online.
Why not start with a brochure-style website, make sure you have access to good stats for the site. Create a few pages featuring products then go to work promoting these online.
The beauty of this approach is that a brochure site has much lower setup costs so you can test the market and find out how much investment (in time or money) it`s going to cost to get a decent SE position. Once you have some solid stats indicating that investment in ecommerce infastructure is going to be worth it, you can confidently make the jump to the higher costs.
I don`t want to prescribe to you what those stats should be, so I`m going to start another thread that asks the question "When are my stats sufficient for ecommerce".
I know this seems like a much slower route to market, but the truth is a lot of ecommerce sites struggle to pay for themselves, so in the interests of stable financials, please think carefully about whether you want to spend the extra money up front.
Remember also that at the moment it sounds like you are quite rightly focussing heavily on how to get an ecommerce system at the lowest price. But there are a lot of important criteria in choosing an ecommerce system, and these will become apparent as you build your online presence.
In conclusion the advantages to this approach are...
1/ You don`t incur costs for an ecommerce site that isn`t yet performing...
2/ You don`t invest in an ecommerce site that doesn`t properly fit your needs.
Double Entry Accounting
The Accounting Equation