Doubts

Pannier are a good idea if you're carrying a lot of stuff. It always takes a bit of getting used to riding fixed, particularly if you've been riding with gears for a long time. I assume you've got at lease one brake on the fixed. Use it to slow you down to a comfortable speed when going down hills before you start to spin out. Also, you'll soon get the hang of resisting the pedals with your legs when you are at hills to make the ride down more manageable. I ride both geared and fixed, but for commuting around London, I find fixed to be a lot more efficient: essentially the bike does exactly what i want it to do. instead of a courier bag, look into a decent rucksack as the weight will be distributed more evenly.