You are likely going to be happiest with a second hand version of a quality saddle, as opposed to a newer cheap saddle.
Fit for horse and rider matters a lot in dressage saddles. Note that dressage saddles have become more and more built up over the past 30 years, and the deeper the seat and the bigger the thigh blocks, the tighter they fit the rider.
The current retail cost of quality saddles is from $4000 on up so you will likely need to go a bit older to find something for $600. You will possibly need the saddle reflocked and definitely adjusted somewhat.
The best brand for you is the one where the tree best fits your horse. A saddle fitter is a big help here. In my local market, older Passiers are a great bargain and fit me and one of my horses. Other horse not so much.
Anyhow it helps to research the various high end brands and see what they offer if you are going to be shopping second hand.
Very very old dressage saddles from the 1970s and 80s may be too hard and flat for anyone to want to ride in these days.
Anyhow, if they fit, anything from the big brands will be satisfactory. Passier, Stubben, County, Albion, Custom, Schleese, etc. If you find an unknown brand you can Google and see what new costs.