Lending and repayment calculations over a portfolio

I wish to confirm the math for my business plan as I have no desire to go bankrupt.

What I need to know is the exact cost of capital each month. This can be created as a list of 'loans' and all have different characteristics:

• Different interest rates

• Some are repayment like a mortgage

• Some are interest only

• Loans all cover different terms/time-frames/amortization periods

• All are paid monthly

Once all details are tabulated and columnized, I can sum the amounts to see the monthly cost and also find the weighted average (if necessary) or simple average interest rate per month, though I am still unsure if averaging the interest rates is correct.

I'm using Excel 2010 in case it has any useful calculations built in.