Although we've stated 4days, depending on our task list at the time, we can most likely have this set up for you within 48hrs (excl weekends and bank holidays!) :)
Helping you create cross-selling modules on product pages is not difficult at all for us (just finishing a similar project now), and we are bound by our data compliance laws, so you know your website's security wont be compromised!
If you don't already have one, we will create a copy of you site and work on that, so please let us know if your site has any restrictions on being copied.
Please also let us know which version of Zencart you're using.