I want to buy a condo in Hua Hin.

My budget is 3,000,000 - 5,000,000 THB.

A one or two bedroom, balcony, gym, pool, and security.

Location: within walking distance of Soi 84, Soi 90, or Baan Khun Por-Foodcourt. Any advice or suggestions?

Can find something for sure in that price range.  But, what one likes you may not.  I would go there and rent or get a hotel for a few weeks and check it out.  Starting now HH gets much busier many Euros coming for 6 months.