Need to add a shop to an existing site rather than build from ground up ala Magento. Any suggestions?
There are lots of “3rd party” carts that you can skin to look like your own site but which are hosted elsewhere. they are ready-made and a very nice option IMHO. Which one you choose depends on what you need, as they vary a lot. Some are particularly good for digital goods, some are quite simple and just link with PayPal, others are more sophisticated. FWIW, my top pics are FoxyCart, FastSpring, E-junkie, Plimus and BigCartel… but there are lots and lots more, if this is a viable solution for you. Some charge by the month, and some just take a cut of your sales.
How many products you plan to publish on your site?
it really depends on your situation… you could go with a free wordpress plugin or you could go with a paid monthly subscription. We need more information. What is you store? Traffic? product?