Dynamic variables

Not like that, no.

Not that you have any need to. Here’s a modified version of your code:

property theMagFolders : {¬ 
{magName:"AA", lastUpdate:""}, ¬ 
{magName:"AL", lastUpdate:""}, ¬ 
{magName:"AR", lastUpdate:""}, ¬ 
{magName:"DR", lastUpdate:""} ¬ 

tell application "Finder" 
set thePath to the folder "merion_image_archive" of folder "Cumulus_Test" 

repeat with magRec in theMagFolders 
set modDate to modification date of the folder magRec's magName of thePath 

if modDate = magRec's lastUpdate then 
display dialog "There have been no changes to the folder " & magRec's magName 
display dialog (modDate as string) & return & "There have been changes to the folder " & magRec's magName 
--do cumulus scripting stuff here 
set magRec's lastUpdate to modDate 
end if 

end repeat