Went down a similar road a while back.
If you only have one free PCI slot (which hopefully is "native" and not "bridged") and no free PCIe slot, then for 16 channels of ADAT connectivity (2x ADAT I/O) your options are pretty much limited to:
A. RME HDSP 9652 PCI (2x ADAT I/O (or 3x with daughtercard))
B. MOTU 424 PCI/PCI-X host card + MOTU 2408 (3x ADAT I/O), or
C. PCI Firewire card (with T.I. chip) + M-Audio Profire Lightbridge (4x ADAT I/O) or Presonus Firestudio Lightpipe (4x ADAT I/O).
The RME and MOTU options have MIDI I/O and are still supported with current drivers and software, but the M-Audio and Presonus FW interfaces may no longer be, dunno. You could always add a USB MIDI interface if necessary.
If you opt for the RME 9652, make certain it's the "HDSP" version and not the earlier "DIGI" or "Hammerfall" (not "Hammerfall DSP" = "HDSP") or "Nuendo" versions.
Fwiw, I went with the MOTU option for ease of expansion and its analog I/O (but with a 424 PCIe card).