What should I consider when applying for a multiple entry non-immigrant ED visa in Thailand?

I've never heard of a multiple entry ED visa. You're required to be attending classes to qualify for an ED so leaving the country is frowned upon. Also an ED visa grants entry for 90 days so that's not much point for multiple entry. If taking informal lessons at a language school you'll need to do an extension every 90 days and apply for a re-entry permit when you get your extension.

If attending university classes you can get a 1 year extension, same thing with the re-entry permit. But once again leaving is frowned upon.

For guarantor, most people put the hotel they'll be staying at so if you already have a landlord lined up then that should be fine.