When you say 'all old files', how many Gb are we talking about? Keep in mind that the storage on the iPad is limited and you'll need to leave a bit of room for temporary files, updates and such.
Wild thought: how about a good NAS that you plug into your router? You can store all your files on it, depending on the type you can even have it back up itself and if you want to access something a simple app on your iPad (something like FileExplorer) will let you browse through all the folders on it. That way the iPad isn't a full PC replacement in itself, but is the 'access point' through which you can get to your documents and if needed open/edit them.