Don’t worry what others think, whatever works for you works. Wear jeans by all means – depends if you find them comfy or not. I wouldn’t get too hung up on the bike. Just buy a cheap half decent one – i.e. not a cheap Halfords Trax effort. I see alot of people at work who are daily all weather commuters and not ‘into’ cycling on Carerra’s and B’Twins and they seem to be perfectly fine and adequate. lightweight enough, half decent reliable kit on them.
Again use flats if you like – if you’re not used to cleat’s then that might be best, but i’m thinking about ditching my road specific pedals and cleats for MTB ones anyway. I really don’t see the point in road specific cleats – MTB ones do the job just as well, you’re able to walk on MTB shoes normally and not like you’re on a sheet of ice,they’re easier to clip in, double sided, usually have a bit more float to them than road systems so better for dodgy knees, stability comes from the stiff sole on the shoe not the cleat. Might be a different story if you’re a Pro, but 99.9% of people are not Pro’s. But then again, no point in clippless pedals at all unless you’re going to use them and actually pull back and up on the pedals.