App Store :: Using Not Actual Versions Of Software

I am trying to understand the concept of the app store. Generally I am not very fond of purchasing apps this way. Not having much experience yet. My biggest concern: 

What if I need an older version of an app. I always only see the actual version in the store. 

For example: I will certainly stay on Snow Leopard for a long time. More and more apps require at least Lion. Once, lets say in 5 years, when I may need to make a new install of all my things (still 10.6.8 tough) how can I be sure to be able to still get the software I always used, in a version that works in 10.6.8? 

I always have copies of apps I use. But in 5 years, will I be able to load it onto a new installed 10.6.8 Mac and run it? Would it only run after App Store confirmed I have actually bought it? Will it confirm a software 5 years old or force me to download y new version? 

Info:
Mac mini, Mac OS X (10.6.8), 2,4 GHz Core Duo 240GB SSD, 8GB RAM