I think the biggest consideration with a freead bun is the unknowns...
First, what if they don't bond? What would you do with her then? Do you have suitable accommodation/finances to keep a second single rabbit until she could be rehomed? Could you bring yourself to rehome her in that circumstance? Or would you end up keeping her then getting another two so that they all have a friend? Could you afford 4?
Second, what of her health? A decent rescue will know their buns inside out and will be up front with you about any potential problems. You may not know if this bun has syphillis, dental problems, mites, snuffles etc until you get her home, and then you will be responsible for all those vets bills.
Third, what about quarantine? You will need somewhere suitable to keep her separate for approximately 6 weeks or more before bonding, so that she can be quarantined, spayed and vaccinated before bonding.
I totally see your argument that you are rescuing her regardless of where she comes from, but I personally think that you're better off getting yours from a rescue where you get a bun ready to bond straight away and backup support if it doesn't work out, and by doing so, it will free up a space at the rescue for them to help out another bun like the one you have been offered