First, thing, If you wanna spend $7,000 I would go straight for a 97-98 900SE Turbo.
And you may not really need to change the O2 sensors. You may just need to take them out and sand some rust off. It could be causing a bad ground.
Exhaust I would go with a nice Stainless unit, that way you needn't worry about replacing it again.
I think for a 7inch wide rim, I would go with the 215/45/17s And find something with a stiff sidewall. I gather that 91 or higher is good.
Also check out this post on SaabCentral. We convinced the guy to get the turbo model for a bit more. And I think he was pleased.