Ok, well, there's two parts to this.
a.) for a Wish to grant your evil party time to buff would like granting them a mass time stop or granting them 5 or 6 spells, in some cases each. Either way, Wish can't do that without having some real issues in the wording. Remember that what's good for your bad guys is good for your party. If you want your party to be able to "buff" in a few seconds, then maybe; however, you possibly are opening yourself up for a lot of discussions in the middle of combat, i.e. "I altered my wish to include spells that I don't have access to but my character has seen cast on others."
My solution would be to avoid the idea completely. Use the top part of the wish spell that is very specific, relatively speaking, compared to the last part.
b.) Your players really shouldn't have a chance in heck of ambushing this group. The evil party has three wishes a day to play with to prepare an ambush site. Let's assume that it takes your party 3 days to reach the Roc. Now, the evil party has 9 wishes to burn to prepare. That could translate into 6 contingencies with three free wishes or just 9 contingencies, i.e. "In the event that Darl gets below 10hp or is affected by limiting effect, cast heal." or "In the event the other party reaches this point or within X number of feet of this point, cast plane shift twice to bring the evil party to the Roc's nest." etc. Lots and lots of evil possibilities.
Look at the ambush from Darl's perspective. He tried the first challenge and failed. He is being followed and is racing to reach the well. Hmmm... Why not go to the last challenge mentioned, do it, cover his tracks, and the set up an ambush to collect the rewards of all of the challenges? The worse that can happen is for one of your party members has arcane sight and sees the aura of the contingency.
Remember that the goal, or mine at least, is to test the party's ability to work as a team to over come a teamwork based threat. Use the monk or monks to grapple and take the clerics holy symbol. Use the horned devil to steal the main warrior's weapon and then toss it to the genie who plane shifts it away. This evil party is smart and has had time to prepare. Either is scary. Both is just deadly.
Since Time Stop is a 9th level spell, you really can't duplicate it with Wish, let alone some sort of Mass Time Stop. And you don't want to, because the party will eat you alive with that later, if you allow it.
You could use wish to create a Contingency that teleports Darl's party away if any enemy suddenly appears close by.
However, what if you have Darl issue a challenge to the party for open battle, giving both sides a chance to buff to their heart's content? It might not be completely characteristic for a Vecnan priest to do this, but hey, maybe possessing the Hand of Vecna has made him a bit overconfident? Or, set it up so that Darl has an ambush set up, but inadvertently telegraphs it to your party somehow, so they have a chance to turn the tables, but only after Darl's group is buffed and ready for action.
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My party managed to defeat Darl fairly easily with Bigby's Crushing Hand. Darl failed his save by, like, 1 or something. So, he was out of the fight for all intents and purposes. So, after they defeated the rest of his group, they decided to interrogate him. I used this opportunity to teleport him away, seeing as how they were going to take the Hand of Vecna away from him. I wanted him to reappear in the final adventure so I just whisked him away. Later, they used gate and pulled him through. This is after they buffed themselves in anticipation of this very event. Needless to say, Darl did not last for a cameo in the next adventure.