[GIBSON]

Good questions, first, with all the modifications you seem prepared to do, why buy an IS? Just look for a good 325i and
make the changes you are considering. You won't get the limited slip the 325iS has but you can find one to instal if you want.
It can be hard to find a clean 325 but they are out there.
The 911 is such a different car that it's hard to compare the two.
The SC is a good car for your first 911 if you find the right car.
There are some really good cars out there but the SC has recently
appreciated quite a bit so they are not cheap. Engine issues can be very expensive and rebuilds quite costly.
So, do you need the room the 325 offers? The trunk, back seat etc. or can you live with the smaller 911. The two cars handle quite differently and you'd want to be sure you like the basic
oversteer of the 911 vs the understeer of the 325. Both are great cars, just very different. Good luck with your choice.