So, yes, you would lose your programs if you installed LMDE instead of LM 12. You can upgrade to LM 13 from 12 without losing programs.
As for your data: you can back up any data in your /home directory, and/or create a separate partition for all your data and not install LM or LMDE to it. Like many others, I have a partition for /root, one for /home, and a third for all my data. Whenever I install a new distribution, it by definition overwrites anything in /root, while you get usually instruct the installer to not reformat/overwrite /home (should you choose). There are times when reformatting/overwriting is a good idea, especially across different distributions, so it's best to save all data in a third partition, and leave /home for personal settings and configuration files.
Hope this helps.