1. If you're thinking Hollywood celebrities for hosting Gear, you're looking in the wrong media. Think about magazine columnists from Motor Trend, Road & Track, Car & Driver. (Heck, even Top Gear writers Jamie Kitman or Pat Devereux could qualify as hosts.)
Why? Columnists as co-presenters (a) could be break out stars, and (b) would be cheaper than established celebrities.
2. Why use a Stig for a tame racing driver? How about getting someone like Danica Patrick instead? All that's needed for a test driver, after all, is racing consistency.
3. I'd nominate the Dodge Avenger as Gear's "reasonably priced car." It has a unique cache in being NASCAR's platform for its Car of Tomorrow, despite being almost universally panned as rubbish to drive. (Which, after all, is the point of the "Star in a Reasonably Priced Car" segment, which is about the Star's ability, not the car.
4. A one-hour time-slot means about 40-44 minutes per episode. If you plan segments around commercials, with a little editing help you can still put together a viewable show.
5. In terms of adapting the format, the "unexpected expense" will come in the form of music for the car segments, due to performance / copyright issues in the US. I'd expect a lot of alternative / prog rock tracks by unknown bands needing the break.
6. And let there be crossovers with Australian and Brit Gear. Things like James May reviewing the upcoming Jaguar XF with a bigger engine -- while ferrying around members of the Jacksonville Jaguars football team, all with something to contribute.