Look at something based on Wordpress and a paid for Wordpress ecommerce plugin/theme. Do not do custom as you will get ripped off.
Also Wordpress allows for SEO guys to quickly add in the required SEO plugins your site will need to get your pages ranked.
You need to also consider a good payment gateway like payu.co.za but do your homework and make sure you don't get locked into any single vendor. There must be scope to add more payment options for your clients.
www.obox-design.com is based in Cape Town and one of the worlds top 10 leading Wordpress theme houses. Check our their stuff and give them a call.
Also make sure your going mobile else go home. Mobile ecommerce is the biggest thing as more people buy using mobile devices like phones and tablets than desktops in South Africa. Don't ignore this market so make sure the site is responsive.
I use to work for a leading SEO company last year and I'm also going to be making my own e-commerce shop for my wife's small business. The above is just a quick summary of my research that I don't mind sharing. You could even pull it off for R10K and your clients will be blown away. Just buy off the shelf components, don't allow someone to BS you into doing everything customas R50K is not enough.