I'd rather use the APPLY operator:

SELECT Equipment.EquipID, x.EquipmentAmount

FROM Equipment

OUTER APPLY

(

SELECT TOP 1 InvoiceEquipment.Amount AS 'EquipmentAmount'

FROM InvoiceEquipment

WHERE InvoiceEquipment.Type = 'A' AND InvoiceEquipment.EquipID = Equipment.EquipID

ORDER BY InvoiceEquipment.InvoiceDate DESC

)x

From my point of view the code is easier to read. And for sure it'll perform better since ther's only one row returned and no need to number all rows just to use the first one...

Edit: An index on the InvoiceEquipment table with EquipID, InvoiceDate and Type together with EquipmentAmount as included column will help, too.