While I do consider 2008's Barca to be Pep's best work, let's not pretend he had a "hard" job. He was coming to a team that had recently won the CL and multiple La Liga's. An underperforming team, but a team with Xavi, Iniesta, Valdes, Puyol, Eto'o and Abidal in their prime. A team with up and coming world class talents in Busquets, Pique and Messi. He also brought Henry to strengthen the side. So yes, Barca were no mugs. In terms of squad talent, Barca and United back then were by far the best so it's logical to assume Barca would win the league and give a good showing in the CL with a competent manager in charge with Pep certainly is.
I don't know what Klopp has refused, but he has certainly delivered the same as Guardiola with much less resources and that's a fact. At Mainz, at Dortmund, at Liverpool he has always punched above his weight. As far as winning CL's and what not, it isn't about that, it's about the fact that Klopp has transformed Liverpool from a mediocre midtable team into title challengers, capable of fully competing with Guardiola's City in very little time and money spent. Yeah, sure, you can argue there is no guarantees that Klopp would do better if he had City or Chelsea budget, but is it likely? Well, yeah. I fail to see why Klopp would fail at those teams if he had the luxury of paying for 50 million duds that would later rot on the bench until he hit that one guy that works for him. Man City probably don't have an unlimited budget, but they're pretty close to it. No other team beside PSG can waste money like Guardiola wastes and continue to compete.