Very strange to hear from a sales rep that distilled water is fine. Storage solution only costs $15. By the time you realize your probe is not accurate you will have used it with incorrect readings for some time.
Check out companies that manufacture scientific equipment like Hanna or Milwaukee and others. Use the specs outlined and you should be able to get one for <$75. You get what you pay for and for how well you maintain it.
It's a good concept but cheaply made. My broke after 48 bottles - the blade came out of the plastic body. I used a $3 putty knife and it worked just as well.