Yes, you can order seperate components if you are an existing customer.
To get an idea of the prices go to the configurator of any systems, note the total price and then change the second drive from 'none' to one of the SSD's, the total price should change by the rough amount of the selected SSD - that's what I do to get a rough idea (noting that it may not be the exact price, but should be roughly correct, I think!)
Yes you can order parts from PCS as a existing customer.
When you do, you will notice on your orders page it will be done as a sub order so you have complete records of everything you have purchased and the invoices etc.
On this subject, I am about to purchase a new computer and was wondering about the merits of including a SSD.
I understand that it will boot faster and be more stable - does anyone have any opinions on whether the additional cost is justified? I am planning on spending approximately £1250 on a computer which can handle the latest games and would like it to be relatively future proof.