specific excel to handle cash flow/debts..

I'm a sole trader in construction and need a system that can handle my expenses and income. With that simplistic frame it doesn't sound too complex.
However, here are some more details that I'd like specifically tailored to my business...

- each fortnight I deduct an 'income' from my business account into my personal account
- at the end of each month I accrue a debt to one supplier that needs to be paid by the following month (30 day terms)
- in the current month customers pay me an initial payment / deposit

I'm looking for a system that i can input ins and outs as they occur so I can see how my current cash flow is and more importantly whether i can meet my debt commitments from my supplier in the following month.

My knowledge of excel is at a stage where I can create some pretty tricky formulas but this has me stumped. Thanks.