Hi Lady G,
Would it really spoil your fun if I even suggest a specific food :D I used to use Wafcol Greyhound food for my pack(why pay VAT if you don't have to?) They do a red bag which is the racing formula and a blue bag, which, if I remember correctly is their maintenance one and is the one you would need I think for your foster dog.
You may have to order it but I was quite happy with this as a complete feed (although I added things to it

)
Christine