I would recommend the Sweetwater series. They would be located at all Sundance dealers. You should be able to pick up a Sweetwater Caymen for between $4,500, and $5,500. This unit can only be on a 220v line. It is a five person spa with a three year warranty.
I would also recommend a Tiger River Bengal, but I would doubt you could get one under $5,000 (It should be in the $5,000 to $6,000 range), however this unit can run on a 110v outlet which will save you some money on wiring your electric. This unit would be sold by all of the Hotspring dealers. It is also a 5 person spa, but it has a 5 year warranty.
Chris H