What type of deal/guarantee was Sollozzo hoping for when he met with Mike at the restaurant?

Probably none. Sol probably was trying to buy time to take another shot at Vito. He needed Vito dead to get the deal he wanted from Sonny and Tom.

Yes, and that's why I was wondering why he decided to meet. He had to have known the Corleones were still seething from the shooting. It would have made more sense to stay in hiding, no? I think if he stayed away long enough, he and Barzini over time could have ended up winning this drug trade business war...

Sol knew that Sonny had lots of people hunting for him. He hoped that, by meeting with Michael, he'd at least put some kind of phony proposal on the table that Sonny would consider--maybe delaying retribution for long enough for Sol to get Vito.

This is a clear sign that Barzini, not Tatt was calling the shots. Tatt lost a son because a cop slapped Michael around a little, yet Sol was dispached to say HE could make the peace among the families. Michael knew the "key" for Sol was to "kill Pop," and that because there was already big money from at least the Tatts, to get into drugs, Sol could not wait all that long to take another shot.
Clearly the meeting with Michael was a ruse to lull the Corleones into a false sense of security.

Sol and Barzini knew Sonny'd get mad after the first hit on Christmas eve. But if Sollozzo killed Vito in a second attempt after the "truce", no-one would be able to stop Sonny. Not Michael, not Tom, no political considerations.
Sollozzo and Barzini must have known that. Which means, they had been prepared for a war already.

Or, if not an all out war, maybe they were figuring out the best way to eliminate just Vito and Sonny? The rest of the Corleone Family would have to fall in line with whatever the other families said anyway.

The Sonny set up was definitely planned out...but I sometimes wondered how long they worked to set that up??