You need the core system at £20.00 per month + the Ecommerce add-on for £10.00 per month - making a total of £30.00 per month. This will give you all the tools you need to get your products online.
A useful addition to ecommerce stores is the 'Frequently Asked Questions' add-on at £2.00 per month. This is good for adding commonly requested information such as 'What shipping service do you use', 'How do you handle returns' etc.
You can take payments securely with Paypal or Sagepay - although you'll need an internet merchant account (available from your bank) to use Sagepay. Each payment provider will take a fee for processing transactions - current rates are available at their respective websites - www.paypal.co.uk and www.sagepay.co.uk