Those older fixed choke barrels have thinner steel and are harder to thread for choke tubes. Unless someone has come up with something different in recent years you won't be able to get it threaded for Remington factory tubes. They used to make a special thin tube to be used for those barrels. You can check into it, because some things may have changed, but I had one done a few years back and had to go that route.
$450 is a good price if you like it as is. I'd not have much use for a 30" full choke barrel and would have to invest in another barrel with choke tubes before I'd be interested. You can find spare Express barrels cheap, but replacement Wingmaster barrels are around $350.
At that price you're into new 870 Wingmaster prices. And I'd rather have a newer gun simply because of the God awful reverse stamped checkering they put on those older guns.