Hello Cymjay, compared to a Buick, the feeling should be of a 'much more stable' car specially at higher speeds..
The 'unstable feel' suggests poor alignment or worn out suspension components, and the vibration (shaking) suggest worn front shock absorbers and/or worn out engine/transmission mounts, how many miles does this car have?
As others suggest, find another Boxter and test drive it again
A Boxter in good condition should feel Very Stable and without any vibrations, now regarding the torque.. unfortunately you cannot compare it with a diesel engine the Boxter engine likes to be (you have to..) reved-up a bit to get you moving but will be very smooth while making beautiful sounds.
If you find one in good condition, you will be hooked :-)