If you are not afraid to experiment a bit, and/or you have a limited budget, let me recommend "Gimp". It is open source software that aims to provide Photoshop like functionality. Photoshop lovers hate it, but I won't enter that religious war, it does what it does.
Anyway, it offers you a channel mixer option, where you can create a B&W image from a single RGB channel, or any custom combination of channels you want (i.e. 80% R + 15% G + 5% B). You can even use negative values, which in some cases it makes sense (e.g., negative blue if you want to darken that bright purple shirt that your model was wearing without affecting the skin tone).
If you go down this route, you might also want to install the "g'mic" plugin and play with its channel mixer. That one allows you to mix in different colorspaces (CMYK, HSV, Lab, RGB and YCbCr).