Whatever you choose, go for something that's aimed at business customers rather than personal customers. The same person will pay more - and with less persuading - for a business expense than for something personal.
With that in mind, try to find a service or product that a business person really needs - that's the key.
Check out this post about choosing a successful online business idea.
The main thing to remember is to choose a subject you absolutely love because your enthusiasm will make your web marketing easier and help win over customers.
WordPress works well for most businesses - you may need to add a plugin or two to customize the output but it's a robust platform. I've worked on websites for major companies that use WordPress.
Don't forget it doesn't have to look like a blog. A good WordPress theme or web designer can make your site look like an online store, portfolio or any other kind of web presence.
The main hidden "cost", which you might not have considered, is the time needed to promote and market your new store. To be successful, you'll need to research the most effective methods and take the time to actually do the work.
Setting up the site is a good first step - and congratulations on getting that far - but effective web marketing will make or break your business. Good luck!
Before anything else, it has to said you deserve a great deal of kudos for getting yourself this far. Most people just think about ideas but never act on them.
Why not go to the type of community you'll go to when you come to marketing your startup? If you look hard enough, there's almost certainly someone there who is also a programmer. Because they share your interest in the area your startup serves they're more likely to get involved.
Because good programmers are forever being asked to code for a percentage without upfront payment, you might want to consider offering some kind of initial payment as persuasion / bribery.
That's a tricky question to answer without knowing the business you're going into or the skills of you and your former partner.
Is there anyone your former partner could recommend? If you have extensive connections on LinkedIn or Twitter, try asking them for recommendations.