Hi,
For testing, here's what I'd start with:
Model: GPT-4o - it handles longer, nuanced conversations well which is exactly what you need for buyers and sellers.
Voice: ElevenLabs - best quality voices on the market, very natural sounding. You can browse their voice library and pick one that fits the tone you're going for.
This combo will get you up and running quickly and gives you a solid baseline to test from. Once you've done a few calls you'll have a much better sense of what to tweak.