Hi Phil,
For custom coding an e-commerce site or coding from scratch, I can imagine it would be a very long process and very time intensive to learn. You mentioned you want a site that the client can update, have you previously built basic CMS sites for clients? As I would guess this would be a good first place to start.
I've previously built a site using an e-commerce package that's been developed for the UK market (
OpenMind Commerce) unfortunately the site is no longer live, but you can see from this screenshot the extent I was able to 'skin/template' the e-commerce system with HTML/CSS -
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Going for an existing e-commerce system might be a more viable route for you? As most tend to include a number of payment gateway modules that can be switched on/off, and a basic site CMS, as well as a more in-depth product CMS. The one thing I would advise, if you chose to go down this route is to get a full specification set, with every fine detail confirmed, as I learnt from experience when the client changes their mind about the delivery pricing structure, to a method/ides that the e-commerce system doesn't support, it can be a nightmare!
You might want to look at other opensource options too, and perhaps even the new arrival on the e-commerce scene,
Magento, you'll need to check how ready that is for the UK market though as last time I checked they were having some VAT problems with the system.
EDIT- Sorry, just seen from another thread that you already know about Magento, will leave that section of my reply just incase its useful for anyone else reading
Hope that helps, and good luck!
Greg