Paranoia Will Destroyia

I am about to have a new website created from the ground up and I was wondering what software I should make sure I secure from the designer/programmer prior to and after I make final payment? The site will use flash, php, Joombla (??). I have heard a method wherein the buyer pays in portions so they can detect any trojan bugs in the scripting/code prior to making final payment. This is all and well if a person can undertsand coding etc to verify if its been compromised or not.

A # of posts I checked out on various webmaster forum sites mentioned that there are many website creators who freelance and after releasing their site to the buyer after final payment many have added hacker programs built into the code for unauthorized access to the sites they just made. I can see this happening all too easily since I would have no clue how to detect if such a programme is in my site coding or not. What safety measuress can I take to handle this issue so I am sure my site has not been compromised when created?

I am a concept person with very little time and interest in my schedule to deal with the actual programming aspects of website creation as I do not have time to deal with the in depth issues required hence I need to put trust in the creator of my new site. I have admin staff to deal with the ecommerce and customer service side and I will eventually hire a site manager to insure the site runs properly as I aim to be a hands off owner in a short time.

Any thoughts on what your experiences are with such would be be much appreciated!

BCBUD