seeking advice on cordless impact wrench

[Gelbster]

or a cheap corded 120v impact from HF. with 20% off coupon.
If you really(??) need a 400ft.lb+ monster, try a 3/4" and an adapter. Such a tool will shatter cheap impact sockets but will always undo/break a nut/bolt. Careful.

[Topless]

This one should serve you well for most automotive work. Just keep a 1/2" breaker bar with helper around for that rare rusty bastard requiring max leverage to set free.