Here is how I went about buying one for a previous SB.
I got on the Cars and Trucks section of Craigslist for my area, found a fairly used car that still had transferable manufacturer warranty left on it, contacted the owner (not a dealer) and asked if he'll be willing to have the car inspected by an independent inspector. Contacted the LemonSquad,
www.lemonsquad.com, for a Premium Inspection of the vehicle. Lemon Squad is ASE Certified and BBB accredited. Once inspection is completed and passed, I met up with seller at a Barnes and Noble where the transaction was finalized (paid in cash). He signed his part on the title but I left the new owner section blank.
Called up SB, gave her the keys, title to sign the new owner section, and a loaded prepaid card enough to cover registration, tax and title fees, and one year of insurance. I just played the middle man - nothing leads back to me that I once "owned" the vehicle.
My SD/SB relationship was very much legit without a doubt but I will make sure as hell not to leave anything to chance/luck, that is leaving any sort of paper trail behind.
As for building her credit, a secured credit line will do the trick. Some bank offers secure credit cards from $300 - $10,000. Do your research as well, a few banks will allow the card to graduate or be unsecured once she builds her credit and qualifies for an unsecured line - she can keep the money that was held to secure the card in the first place. Win - Win for her. Most banks will not give back the money unless you close the account which could in turn hurt her credit again.