Hi Ana,
Hope you have a SAP All authorization User Id to check the T-codes.
Coming to your Qns;
-how to find out the output devices for a specif printer
If you are printing the invoices through VF01/02/03, in the pop-up screen you are selecting the Output device. For knowing the printer details you can go to SPAD. Check the below links;
http://scn.sap.com/thread/978153
http://help.sap.com/saphelp_nw04/helpdata/EN/d9/4a8eb751ea11d189570000e829fbbd/content.htm
http://help.sap.com/saphelp_nw04/helpdata/en/d9/4a971d51ea11d189570000e829fbbd/content.htm
-how to find out spool numbers
In the T-code SP01, after taking the print-outs you can see the Spool numbers, if you are not able to see, change the user name as blank and give an older date period, in the screen and execute the T-code once again.
- the report with the invoices is called : ZVINVPR3 where can i get further information about this report?
You can go to SE38 T-code and give the program name there. With the help of an ABAPer please do the analysis 
Hope this will help you 
Thanks & Regards
Manu S