I have thick, coarse wavy hair and find if I look after it well, my BBD lasts at least 12 weeks, if not longer. I am a complete convert, they're brilliant. You need to use a shampoo without sulfates (although you can use any conditioner).
Your hair may go greasy more quickly. I used to wash my fair twice a week, then found with the BBD that I needed to do it twice a week. however, the more you wash your hair the shorter the BBD lasts. so, now I use dry shampoo and leave it an extra day before washing.
I wash with Moroccan Oil shampoo but have also used the L'Oreal , Organix and Naked sulfate free hampoos which are all fine (and a lot cheaper... I just love the smell of Moroccan Oil). You can use any old conditioner. I run a small splodge of Moroccan oil in my hair before blow drying. Sometimes finish off with straighteners if I want it dead straight.
It won't give you straight hair, but it will give you more manageable, shinier hair which takes about half the time to blow dry. You can leave it to dry naturally as well, although it might be slightly more curly.
You can get your colour and BBD done on the same day - I did that with the last one I had. however, if you're normally cherry red it might lighten the colour a bit so you might want to go a shade or two darker. Ask the hairdresser.
Check with the salon which brand they use and make sure it's a formaldehyde free version.