OK, You have a 20Mpx image. Lets do some math.
18" at 300 ppi
54" at 100 ppi
108" at 50 ppi
50 ppi is TOTALLY FINE for a photo that big.
That is a pixel arround of half a milimeter.
If you are really paranoid Xo) you could double the resolution with a bicubic Sharper option on Photoshop. I mean 200% (Not 123.456% or anithing else) Perhaphs you can aply some sharpen after the resampling.
Yes, you could rent a half million dollar camera... you will only get a picture of 80Mpx, probably a sharper image, but you will only get 100 ppi at that size.
Just remember to make your texts and logos in vectors.