I'm still of the "'83 Trek 400 is worth $300" school, and assuming it's in good condition, I think your '734 is well worth it despite the component swap .... (I've seen my '84 Trek 760 raw frame alone sell on eBay for $300+.)

It's too bad though, that the original Suntour Superbe was replaced: I've never heard of "Sport", but I have read that both Sprint and Spirt are a step down from Superbe, and I'd say this reduces the value of the whole package significantly.

- The Spirt FD has a peculiarity that most people especially dislike: it operates in the reverse of normal (big vs little chain wheel).

Finding a replacement Suntour Superbe group would set you back quite a bit:
I would keep that in mind, see what is actually is on the bike, and negotiate accordingly.