There's a program called the Amazing Slow Downer, which slows down,songs without changing pitch.
Again, paid and unpaid version. I have a version which came with a play a long book from Hal Leonard the publisher. Will only work with Hal Leonard backing track. Never used it.
However, others swear by it.
A simple search on the net would find details.
I started out on a Hohner Special20, but gravitated pretty quickly to a Hohner Marine Band 1896. It just feels more comfortable, despite a range of issues re maintenance.
I have a whole heap of other harps - Suzuki Manji's Seydel 1847s and Orchestras, but I keep going back to the Marine Bands. I occassionaly have splashed out on the more expensive ones - the Marine Band Deluxe is my current favourite, but I also have a few Crossovers, which are good.
If you like the harp, that's all you need to worry about.
Here's another attempt at 'Til There was you.
Again, like playing Begin the Beguine on this low Eb Seydel 1847, slower felt better (plus someone wanted me to play it slower anyway).