The first question would be is your slug barrel rifled? If that is case then it has no choke and will not pattern buckshot. If it is a smooth bore it is an I/c choke and is going to pattern the same as any 18.5" I/c barrel you get.
Next is since money is an issue then the Remington factory barrel - 18.5 Parkerized Police barrel with bead sight and a fixed I/C choke is the best value out there at $101 on the Remington parts list or as little as $98 from Remington parts Dealers (like me)
Or, if he has to match the Express finish then the same barrel in the Express finish is $134 from Remington.
Since you want an HD barrel, pattern is not important at close ranges found inside homes and building so getting another barrel is not an issue if your 20" barrel is a smooth bore. If you want a tighter pattern then there is an $18.5 that is threaded for Rem Choke and comes in a matte finish with one modified tube and sells for $148 or an 18.5" police barrel, parkerized with factory installed Meprolight tritium bead and a fixed Modified choke for $148. You can find all these barrels on the accessories page of www.aiptactical.com
and they are less then factory prices and some are factory new Gun Smith Take offs and are far less, for example, the 185 matte is $134 from Remington but $120 on that web site as it is a Gun Smith take off.
But again, if your slug barrel is smooth bore then don't waste money on another barrel.