What’s a sensible impact wrench for general car work?

I’d look at a mid-torque 1/2-inch model first, such as the 891 or 892. It’s a useful middle ground for general automotive work without jumping straight to the size and power of the 961. If a fastener is badly rusted or unusually tight, even a mid-torque tool can have limits, so keep a breaker bar handy rather than expecting one wrench to handle absolutely everything.

If you want one tool, I’d lean toward a mid-torque rather than a compact 3/8-inch wrench. A compact one can cover a lot of ordinary jobs and is easier to fit into tight spaces, but stubborn wheel or suspension fasteners may call for more muscle. The tradeoff is pretty simple: smaller and lighter for most jobs, or mid-torque for better odds on the difficult ones.

The 961 sounds like a lot of wrench for a hobby car unless you regularly run into very seized fasteners. A 900 is another option if you want more power than a mid-torque while keeping it more manageable than the biggest model. If you only want one impact, though, I’d still start with the mid-torque and use hand tools when something refuses to move.