I would lean towards used, personally, since some of the cheap synthetic saddles can put you in a strange position. For a reasonable used dressage saddle I would expect to pay around $500-$1000. The exact brand will depend on personal taste more than anything, but Karl Niedersuss and Passier are two that I’ve seen hold up with a lot of use.
If you’re not sure about a brand, try googling reviews for that model - some of the lesser-known brands aren’t known for a reason, but there’s quite a few smaller makers that make good quality saddles that just haven’t been seen by a lot of people. Those brands will often come cheaper since they’re not the ‘in’ thing.
Do you have a tack store nearby that sells saddles on consignment? We have one that lets you take multiple saddles on trial, so you can test a few and decide what fits you & your horse the best, and obviously they sell for a little cheaper than a brand new saddle. If you know any dressage or event riders, you could ask them if you could try their saddle to see if it fits, and then look for the models you like on ebay. I think it’s worth looking around for a saddle that your mare really likes since there’s so much emphasis in dressage on movement and relaxation, and having a saddle that’s uncomfortable for her can really make those difficult.
If you have a dressage trainer, it would be useful to have them critique how you ride in a few trial saddles until you know what position you’re supposed to be in. You might want to consider measuring your femur to find out your saddle size; 16" is generally on the small side for someone who’s 5’7.
Half pads are only really needed if the saddle is slightly too wide and/or doesn’t provide enough wither clearance. They are in style at the moment, but if your saddle fits correctly and your mare doesn’t have an excessively sensitive back, you don’t need to get one unless you like the look of them.