I personally like to keep things as simple as possible. Since this is being done on "downtime", that seems reasonable; the whole job thing isn't a major focus of the game, just something that's being done on the side for a bit of character development and possibly a minor benefit occasionally.
My own personal approach is to skip creating new rules when I can hijack something that's already around; especially since already existing mechanics will often have plenty of people that complain about what's wrong with them and how to fix them.
Basically, why bother creating rules when there's already stuff playtested and bitched about just waiting for you?
In this particular case, I'd look to d20 Modern actually. The index page for what I'd look to use is here:
d20 Modern System Reference Document
Now, I'd personally treat any of the business venture stuff in D&D abstractly, simply using the Wealth mechanic from d20 Modern:
MSRD Wealth
Essentially the business can be off doing its own "thing" as it were. If a player wants to tap into the "profits" of the business in order to acquire gear, then it's a Wealth check. Failure indicates that there's not enough resources left over from the operation of the business in order to purchase whatever they want.
This is kept seperate from the cash that's given to a PC, although they can convert the cash (gold, magic items, whatever) over and roll it into their business if they want; I'd probably encourage the player to keep them seperate though, with the treasure given during the course of the adventure simply representing the easy money and cash-on-hand like it normally does.
You can also consider using the Reputation mechanic out of d20 Modern as well:
MSRD Reputation
The Reputation could be a two-way street. Being an adventurer could help the business, or the business could actually enhance the character's Reputation.
The rules will probably need to be adjusted a bit to fit individual GMs games, but that should be a solid enough starting point for you.
The thing to remember with D&D is that for better or worse, money is a resource that is managed as much (if not more so) at the player level than at the character level; money is able to buy very real game mechanic benefits. That being the case, treating a business as a concrete sort of thing like the money that is awarded for going out and killing things means that you're running a risk of throwing the metagame out of whack.
Abstract the business, allow it to provide some benefits like acquiring mundane gear or even very occasionally a magic item, but mainly leave it doing "in-game" stuff and enhancing reputation or vague things like the the quality of inn the character can afford to stay in when they adventure.