Grim, Mk's is excellent for GT3, There's a downloadable copy somewhere on this GT Site, Do you have a device to transfer game saves to your memory card? I could just send you my save and you just keep the hidden cars you want. MK's will let you build the cars how you want, You will still need a device to transer and save to your memory card.
From this site, Download MK's program, Then use the Save I sent you and load the garage into it, click the "set view" option in MK's and it will give you the cars codes in CB format!