i've been getting a lot of ribbing from friends about john hart... i've been working on a jack harkness costume, and people either say "jack who?" or "uh ho, looks like you'll be kissing spike at the next con!"
despite all that, i think the john hart costume is great!
depending on how much work you want to put in, i'd bet you could pick up a cheap airsoft colt and modify it heavily. you might find a gun that needs a lot more work for way less $$ than something closer...
i do think that the colt pistols look like they'd be a great base.
but, on a ever-so-slightly spoilery note, there's another scene where he carries a pair of unmodified MP5-Ks.