You could first gather all installed PS versions, something like this
property minimumPSVersion : 2
property maximumPSVersion : 6
set installedPSVersions to {}
set applicationFolder to path to applications folder as text
repeat with i from minimumPSVersion to maximumPSVersion
set currentPhotoshop to "Adobe Photoshop CS" & i
set currentPSAppPath to applicationFolder & currentPhotoshop & ":" & currentPhotoshop & ".app"
tell application "Finder"
if exists file currentPSAppPath then set end of installedPSVersions to currentPSAppPath
end tell
end repeat
set numberOfInstalledPSVersions to count installedPSVersions
if numberOfInstalledPSVersions = 0 then
-- error handling no Photoshop installed
else if numberOfInstalledPSVersions = 1 then
set currentPhotoshop to item 1 of installedPSVersions
else
-- choose from list installedPSVersions
end if