Looking up the value for MS Office Interop constants rather than hard coding them

Use the primary interop assembly for Excel. If you have Office installed these should be in the GAC. Use like so:

Add-Type -AssemblyName Microsoft.Office.Interop.Excel
[int][Microsoft.Office.Interop.Excel.Constants]::xlDoubleQuote

Keith already gave you the answer, here's another option. You can use tab completion on the $xlConstants object to get the constants:

$xl = New-Object -ComObject Excel.Application
$constants = $xl.gettype().assembly.getexportedtypes() | where-object {$_.IsEnum -and $_.name -eq 'constants'}

$pso = new-object psobject
[enum]::getNames($constants) | foreach { $pso | Add-Member -MemberType NoteProperty $_ ($constants::$_) }
$xlConstants = $pso