I had a cheap one, made by Cummins Power Tools, supposedly 350lb-ft, cost about $100. It worked fine, but only had enough power to take off lugnuts and that's about it. In reality it was good for about 100lb-ft tops, I'd say. Similar my experience with Harbor Freight impact and a Craftsman one (reliable, but not that powerful)....
I have an older-model Snap-on impact now (bought used for about $200, new they're about $400) and it is "rated" for 400lb-ft IIRC, and I would think it comes fairly close. Anyhow, it will undo ANY bolt, anytime, any place. It's taken off rusted main hub nuts, big rusty chassis bolts on the jeep, and makes quick work of taking off lugnuts. Battery power is good (I have 2) and the charger is fast. It has been used hundreds of times and still works great...been abused, dropped, used as a hammer a few times

, etc.
The moral: buy a really good one (Snap-On is the best, IMO) once, or buy a few cheap ones until you figure out they can't do what you want them to do and end up buying a good one later.
One of the few things where you get what you pay for!