I can't speak for a Brompton, specifically, but a friend has a Bike Friday folding bike, and I am really impressed! Never having seen a folder before, I thought of it as sort of gimmicky...like an "emergency bike". After one 60 mile Sunday ride with the group, I saw that folding bikes are not a joke! It is a "FOR REAL" bicycle!
On your other points, I have to agree with what others have said. Try the commute on a weekend, when you aren't pressed for time. Consider the bike one way, bus/car home route for a couple weeks. Install bar ends, if you decide on a bike that doesn't have drop bars. Everyone won't agree, but I recommend carrying your lunch bucket or clothing on a rack...not in a backpack.
Once cycling to work becomes part of your daily routine, you'll really miss it on the days when you can't ride, due to picking up the kids after work, or appointment, or whatever. Eventually, you won't even want to drive your car for ANY reason! The daily commute may become the highlight of your day!