If you are selling things that can be described with a name and a couple of parameters (size and colour) then you can speed things up allot by using an out of the box shop solution. Make a search for hosted web shop, if you are in the UK then EKM Power shop would be an option. You do loose some flexibility of course, but you gain speed to market and reliability.
If you need more flexibility then consider an out of the box shop such as Magento. There are plenty of companies around that will set it up and customise it for you - but it sounds as though you could handle the coding yourself. I just had a shop that for selling prescription glasses built over Magento - you need allot of variables to specify a pair of glasses - but it can be done for less than £1000.
Graphic design will cost as much as you can spend! One of customer designs probably start at £1000. Or go and see if templatemonster.com has something suitable for 10% of that.
You should put some effort into the SEO of your site, and make it a habit to review user paths and fall of. Google Analytics will do the job, but you'll need to set time aside to do the analyses and respond to it. Perhaps you can outsource the number crunching?
What else! Well a domain, stock, mechanism for fulfilment (could be boxes and stamps or could be outsourced if the volumes justify it.
A mechanism for receiving payment - PayPal pro is easy to get going with, but their customer service, at least for vendors, sucks. Don't forget your SSL certificates to allow you to take card details securely.
And there is the marketing - building a better mouse doesn't bring folk to your door. Because they don't know where it is. In my opinion marketing can make or break a startup - so decide how you'll do it and budget accordingly.
Given £50k budget you could build a site that is capable of being personalised for every customer with integrated online marketing tools. But I'm guessing that since you are asking the question you're not in that space! No offence implied.
So here is my list:
- Domain registration
- Site build / cart licence costs (yes you could bulid and debug your own but why bother?)
- Graphic design
- Hosting fees
- Email marketing engine
- Site analytics
- SEO work
- Card handling
- SSL certs
- Trusted retailer certs (here in Denmar 10000 DKK)
- Marketing budget