Hey Stuntshell!
Hope you are well.
I'm assuming that they want to keep their existing PBX? Ordinarily, I would attempt to get them to bin it and just shove SARK on the T1. Asterisk is brilliant with T1/E1 circuits. The latest Digium cards with Octasic EC are simply awesome. I have never heard better sound quality, even on big, full-house Definity and Ericson gear
However, having said that..
We have two SARK/Trad PBX hybrids running in production; one is with a Mitel switch and the other is with an Avaya Index. In the case of the Mitel, the SARK box is upstream and connected to the Mitel over an E/1 (could just as easily be T1). For the Index, the SARK is downstream being fed by a pair of ISDN2e cards (4 channels). Both set-ups work pretty well but they did take a fair bit of setup and some trial and error to get them to running smoothly.
I personally would not install any analogue gear (TDM400/800 etc etc) if I could avoid it because it's always finicky and even at best, does not give great sound quality, at least, not when compared to digital set-ups.
So, given the choice, I guess I would go upstream of the PBX. You can then "T" in between the PBX and the PSTN with your TE205 (SAIL will autosense whether the jumpers are set to T1 or E1 and set the trunks up automatically). We may have to do a litlle fix for you to handle a third Zap Group (G3), and you'll probably need a few custom apps, but nothing too drastic. Finally, if your customer has the budget, use a 207 (with the Octasic EC). We use 'em all the time and we love 'em.

Kind Regards
S