I'd have the same recommendation for anyone considering renting a property - get here first and spend a month in an apartment before committing to any long-term accommodation rental.
Renting off a website, paying a deposit etc is not good practice in my opinion. Check out the neighbourhoods, locality in relation the shops, beaches, transport etc.
And what are the neighbours like? 3 barking dogs? Is there an all-night karaoke bar or bar/restaurant nearby?
Things like this you won't see mentioned on a real estate agent's website.
You'll get some indication of prices from the real estate section of the local news site:
Hua Hin Today
but I expect finding your own property to rent will be cheaper than the rates charged via a letting company.