Merc buying Brawn was possible first and foremost because Honda stepped out. If that never happened, then there wouldn't really be a team available except for perhaps Force India, but it's questionable they were open back then for being bought. The spygate scandal obviously influenced Merc's reputation, but the damage was done anyway and nothing will change that, so that's just talk, but not an actual reason. Mercedes wanted more name, and iirc there was friction due to the Mercedes SLR situation on top of that.
Yes, Ron Dennis did not want to sell Mclaren to Mercedes, which meant they wouldn't carry the 'crown' if they achieve success, but rather Mclaren, and they (Merc) wanted more. But It is very defendable that Ron didn't want to sell, and he's right, and he shouldn't have. After all, if Mercedes bought a majority stake, it's pretty clear they would have simply changed the team name into Mercedes GP, and that would have been the end of Mclaren right there and then. It's also said Ron Dennis sold a stake of Mclaren to a Bahraini consortium without even offering Mercedes themselves that stake, which didn't got taken well by Mercedes at the time. True or not, it is ironic that that very same Bahraini consortium not too long after actually got rid of Ron Dennis, effectively ending the MP4 project right there. But, in any case, Mclaren as a name and team was saved.
The truth remains, if Honda had not stepped out, there would never been a suitable team available for Merc to buy anyway. Especially not a proven capable team, nor for a reasonable amount of money. If, for the 2009 season, Brawn had bought Renault engines or Ferrari engines instead of Mercedes engines, i even less think that Mercedes would have bought the team. Yes, HRT was an option, aswell as Manor. But again, these were not proven, capable teams, and I don't think the risk involved would have been acceptable for Mercedes. Had Brawn taken Ferrari engines, perhaps Brawn could have actually made Alfa Romeo happen a few years earlier, who knows. With Renault engines, that's a big question, as Renault just started their own team again taking over Genii, so that ship sailed. In any case, Mercedes was the right decision.
In other words, it was very much thanks to Brawn that Mercedes bought the team. Offcourse Brawn knew Mercedes was looking around, and he benefitted from that, sure.
But perhaps another interesting question could also be: what if Brawn always intended to do things like this?
Could his RedBull plan in 2008 actually have encompassed in turning RedBull into Mercedes GP? If you think of it, Brawn turning a energy drinks company few people ever believed in (back then) into a motorsport heritage racing team, with Alonso as the hungry double WDC as the 'Nr1' driver, and having a desired German in the team in the shape of Sebastian Vettel, and run the double-deck diffuser car of Newey with Mercedes power to the dominance we saw Vettel have with the team from 2010 onwards till 2014?
And when that fell through - either because Alonso didn't want to go with it, or, perhaps more believable: Mercedes did not want Alonso due to the Spygate scandal and that might actually be why that 'project' fell through - could he actually have been informed that Honda was planning to stop their F1 project anyway (and buy the Honda team for much cheaper), and instead of Alonso paired to Vettel (of which back then nobody would expect his RB dominance) grab a German in the shape of Rosberg and plan on getting/luring Schumacher as an even better alternative than the RedBull 'plan' with Alonso, perhaps even having planned Hamilton as far back as 2008?
It's all speculation, let that be clear. But something tells me Brawn had some rabbits in his hat.
Dumpster sounds so much more classy. It's the diamond of the cesspools.