First we don't know what version of the Mac OS you have. "iOS 6" in your profile runs only on Apple's iDevices, not Mac computers. Doing "About this Mac" from your Apple menu will reveal your true OS version.
The reason for asking is that, as the Mac OS evolves, it becomes better at defending itself. If you are at, say, OS 10.6.X, you do not have the built-in defenses that the current OS version 10.9 has. Upgrading from 10.6 to 10.9 is a 5.3GB download so you will definitely exceed your limit. If you are already at OS 10.8.X, you have most of the defenses in OS10.9 and could wait until you can get to a faster connection.
Sorry, my Macs are fully updated and I don't remember the size of the security updates I've applied. Here's one for 10.6 that seems representative to my recollection:
It is over 300MB
do the pros recommend replacing the apple security with an outsider ( for example kaspersky ) ?
The people around here, many being pros, recommend you run as fast as possible away from most payware Mac security and cleaning products. We feel they are foisted on people switching from Windows who have been frightened (often rightly so) into the need for extra protection. Think of it as having a high-tech and secure bank vault and someone recommending you buy a pit bull and tie him to the vault door. The developers of such software for Mac commonly use marketing techniques and claims that are borderline fraud.
has some great resource to help you dcide if you need extra protection and, if so, what works with Mac.
To ensure your system is fully updated you ought to download and install the Mac OS X 10.6.8 Update Combo v1.1 which is 1.09 GB. You won't be able to install the "security update" you linked until you update OS X to 10.6.8 anyway. The Combo update may include it already, I don't recall, but you should definitely install all recommended Apple software updates.
Mavericks is an OS X upgrade. It is the one that's 5.31 GB in size.
No DVD installers are available. If your data plan is limited to 2 GB per month, use a public Wi-Fi such as one in any Apple Store or many public libraries or coffee shops.
Kaspersky is junk, at least their Mac version is, which is the only one I'm familiar with. Avoid it.