Can't say if there is anything already on your Mac which lets you read and write Excel documents but I don't think that there is - I use Office for Mac on mine. I think that if you don't want to spend the money then Open Office (
www.openoffice.org) provide a Mac version for free but I have never tried it. This should be compatible with Word and Excel.
I use a NetGear wireless router for mine but am thinking of getting an Airport instead so that I can hook up a hard drive to it and use it as a backup drive and to share music across my 2 Macs.
Macs can read and write to drives formatted in FAT32 but can only read NTFS formatted drives. I don't think that PCs can read Mac drives. Therefore if you want to transfer files you need to do it all from the Mac end (and if your PC is NTFS formatted then you probably (and I can't say if this will work) need to install a FAT32 formatted partition and use that as a conduit for the files).
I think that external storage is probably similar to the above.
Sorry most of this is "probablys" but as mentioned above I've got 2 Macs so not all that much experience in sharing between Mac and PC other than my initial data transfer when I moved to Mac!