Reading your requirements, I have done a brief research on the api, I have found that Quickbooks support following functions via API
Account,
Attachable,
Batch,
Bill,
BillPayment,
Budget,
ChangeDataCapture,
Class,
CompanyInfo,
CreditMemo,
Customer,
Department,
Deposit,
Employee,
Estimate,
Invoice,
Item,
JournalEntry,
Payment,
PaymentMethod,
Preferences,
Purchase,
PurchaseOrder,
RefundReceipt,
Reports,
SalesReceipt,
TaxAgency,
TaxCode,
TaxRate,
TaxService,
Term,
TimeActivity,
Transfer,
Vendor,
VendorCredit.
I can develop different input options for csv files as per the function above. Suppose you can upload employee details, company details, item details by different csv files. All using single interface. Choose me to discuss more.