Excel - Complex Lookup

I have a situation where I want to be able track the addition of new
equipment which wil be entered in one worksheet with the equipment name
(dropdown list), months to build, month placed in service, and business unit
(dropdown list). Each business unit has a worksheet in another workbook that
lists number of equipment units in service in any given month. The day rate
charged for each new piece of equipment placed into service is different in
months 1,2 and 3 (months beyond 3 are the same as 3). So I need to be able to
look up the number of units added for a particular business unit from that
unit's worksheet (there may be more than one in the list of new equipment and
they may not be grouped by equipment name of business unit) and determine how
many are being added for each month and which month they are added in if they
were added this year so I can calculate the appropriate revenue.