I have to ask, what's wrong with the Oppo? Is it dead, or do you just want an upgrade? If dead, Oppo still repairs them, if there's an interest in getting it running? But, if it's still fully functional, why replace it with the Sony UBP-X700? Run a new DAC using the Oppo. That'll give you some extra money for the DAC.
As to DACs, the Oppo uses the well regarded ESS ES9038PRO converters. The Topping D90 uses AKM AK4499 converter chips. Both of those are top of the line chips. Both, I believe, are Delta-Sigma designs. So, I do wonder if the D90 will be an upgrade or a lateral move? If you want a true upgrade, that would benefit CD Audio, consider a R2R Ladder DAC. Many think they're more musical, and cast a larger, more realistic soundstage, than a Delta-Sigma DAC. As an example, I have gear with ESS and AKM chips, and I think my Multibit Schiit Bifrost R2R betters them, for creating a soundstage with good depth. As such, maybe consider Schiit
Modi Multibit or
Bifrost 2 DACs. They are in the same price range as the Topping.
Oh, the Sansui should be fine. It looks to have an Aux input, and 2 Tape In inputs, either which will work, for an outboard DAC.
A 2nd Oh, just curious, do you use SACDs? The Oppo and Sony X700 support playback of SACD. The Topping will decode DSD, but the Schiit won't. Mike Moffat of Schiit considers it a dead format, and exhibits no love for it. If you go with Schiit, you'd have to have your player output PCM digital for a Schiit DAC to properly play a SACD.
'Luck.