(Full disclosure: I work for Stellar.org)
I'm not quite sure what you mean by "We also are based on the lets system with equal positive and negative balances.", so I cannot comment on how that effects Stellar's viability for your problem, but I can talk about the transaction fee you would require.
As of now (December 2015), the stellar network itself only has support for the native transaction fees used for spam prevention. This means that you would have to implement your own support for transaction fees. Let me describe how one such system might work on the stellar network:
Since the stellar network doesn't have support for collecting a transaction fee at the time of payment, you must instead collect transaction fees after the fact. Members of this system would be invoiced by you with their billed fees, and they would pay to keep their account in good standing. When an account is not in good standing, your system would revoke access to your currency (see "Controlling asset holders" at https://www.stellar.org/developers/learn/concepts/assets.html for details) that would prevent the account from making or receiving payments in your currency until you restore access.
Now, such a system is definitely more complicated than if the stellar network handled custom transaction fees natively, but our intention is to keep the core stellar network as simple as possible, and one of the features that got chopped was support for asset-based transaction fees. Luckily, this is a great opportunity for a group of developers to build a helper service to support this use case. Tracking payments on the stellar network and managing an account's standing based upon a set of customizable business rules seems like a generically useful tool to have available for developers and could benefit a large range of use cases, from custom currencies to e-commerce systems, to even something crazy like a stellar-funded metered power grid. I'd be happy to discuss such a solution further if you're interested.