Staying in OS 9 is going to severely limit your options as you probably well understand. You can use FCP 3.02 and DVD Studio Pro 1.5 in OS 9 but only iDVD2 will work in OS 9.
You have cut out the possibilities of going to FCP4 , DVDSP 2 and iDVD3 with out embracing OS X.
If you are doing video as a business, not a hobb;. then you can not afford to relegate your services to old capabilities. I'd also suggest that you find a way to load OS X on your system and get acquainted with the power it offers. That doesn't mean you can not keep FCP 3 on another drive/partition to use in OS 9 in the mean time, but if you continue to ignore the realities of where Apple software is going, one day you will wake up in a corner.
As far as the DVD burner hardware, get one of the latest Pioneer DVD+- A06's or the comparable Sony DVD+- since Apple seems to be shipping both in their latest systems. If you are capable of swapping out the DVD reader, then you can use it with iDVD2. Adding an external Firewire unit will not allow you to use iDVD with it in OS 9 or OS X.