I have run campaigns where the PCs ended up owning shops, bars, or other businesses. Here are some tips regarding your specific questions.
How difficult did you find it to manage the balance between business
This really depends on the size of your group, but in general you don't want to get bogged down in the nitty gritty details of running a business.
If you have 3 or fewer players, it should be easy to make their business part of the adventure. There could be competitors trying to assassinate the PC, a thieves' guild trying to run a protection racket on the shop, or the PC could go adventuring for rare ingredients to make a better product. These things are much more interesting to play out than running the mundane tasks of a business.
For larger groups, its a bit harder because the rest of the players may be sitting around while you play out these scenarios. Therefore, you should do them less frequently, and have the business run in the background.
Did it happen by chance for your group, or was it all part of the plan
(either in your mind or the group)
For my groups it usually happens by chance. I usually play a more improv type game though. I have a general concept of the campaign story, but the PCs can do whatever they want. I still usually try to plan before each session though, so once I know a PC wants a bar, I'll plan some encounters that will let him acheive that goal.
Has anyone drawn up any tables for running costs, ware & tear etc.?
I don't think there are any tables for this, but I'd advise you not to spend too much time thinking about it. You can simply use the PC's profession skill and give him modifiers based on the circumstances. If it's a new business, start with a -8 modifier. If he bought an existing business, adjust the modifier based on how popular it is. If the thieve's guild is running a protection racket on the shop, apply an additional -2 modifier. If he acquired secret ingredients to improve the product, apply a + 2 modifier. More adventures lead to more bonuses.
Additionally, once per month he can spend 50 gp to promote in a new part of town. A successful diplomacy check means the promotion worked, and he now gets a +1 modifier to his profession checks. You can tweak that, but it should get you started. Remember to keep it simple.