Limited company contract

I own a Limited Company and I need a contract in place that I give to my clients which states how many hours I work for them in a month and if they go over the hours I will invoice in addition (this needs to be edited for each client) and approval needs to be given if we work out of core hours - I have staff that work for me so it needs to be stated in the contract they will also complete work on my behalf. We also work from our own office and will travel as required (with prior agreement)...and all the usual confidentiality wording etc (and anything else that you feel is standard/mandatory). I also want it to state that if they do not pay my invoice within 14 days interest will be added daily until they do so.

I'm not very good at legal wording so I'm looking for someone really experienced to do this for me.

I really don't know what the going rate is for this kind of work?