[Sirtel]

Yes, it's possible to use the PW entirely with sideloaded books, without wifi. However, to use collections on the device, it must be registered to an Amazon account. Wifi may be kept off after registering for as long as one wants.

If you don't want to register, then be warned that you cannot sort your books into categories on the PW. There are ways to manage your library, though, for example making a calibre catalog and sideloading it to the Kindle.

The PW should be easy to use for an ereader novice.

I don't think that having a large number of books on the device seriously degrades the performance of any modern ereader. I have about 3,000 on my 32GB Oasis and it has not slowed in the slightest. Kindles index their books after loading, however, unlike Kobos. So it's not really a good idea to dump thousands of books on the Kindle in one go. Load them in batches, a couple hundred each. After loading a batch keep the PW connected to a wall charger until the indexing process is finished. You can check this by entering a nonsense search string on the home page. If there are unindexed books, it will say so. Load the next batch when the PW has finished indexing the previous batch.

Lastly, if you sideload from Calibre, you should use the AZW3 format, not mobi. AZW3 is more modern and supports more features (the boldness adjustment and custom fonts, for example, neither of which is supported by mobi).