Backup windows PC to time capsule. I'm looking to buy a time capsule to back up my MacBook, however currently there are more Windows machines in my home than macs. Os it possible to use some of the time capsule for windows backups?

Or at least plugin an external hard drive to the Time Capsule?
Sure - Time Capsule shares the internal drive and any external drives just as if they are being served on a PC file server using SMB share technology.

You're on your own to select whatever backup software you want that will work with a network shared drive, but there's nothing added to prevent this sort of use in addition to the normal file sharing.