The '"C" compiler and development tools' is a package in the package manager. Download and install it using the buttons in the package manager.
You'll probably also need to download and install the "Infozip" package (it is at the end of the list of available packages)
Otherwise, if you attempt to install the ssmtp package it will let you know if the gunzip program has not yet been installed.
Then you can re-select the ssmtp mail package and install it. It should then compile. You may want to install the swap-file package if you only have 512 Meg of RAM, as you might not be able to compile otherwise without running out of RAM and causing the kernel to start killing off processes in an attempt to free some for itself.
Don't forget to edit the fields on the ssmtp package for your e-mail provider.
Does this need to be set to reinstall every reboot? The reason I ask is well I did and I was tinkering around with my case since I wasn't happy with the wire layout and I'm trying to figure out why one of my drives is acting up. Anyways I had everything set to reinstall and it almost appeared after reboot I coudn't get an ip address then I couldn't even view the contents of /boot from command line. It was like my machine went a little bonkers. I more or less erased my USB stick and started over. I don't have tons of time invested, but you know... just trying to figure out things and my data is protected either way.