I agree with Sara...out of those choices, I'd pick the older, used Collegiate as well. Back when Miller's distributed Collegiate, they were fairly similar to the Miller's Crosby saddles. Not quite as nice, but well balanced and durable.
I would not recommend the HDR. My friend bought an HDR Advantage and it was so cheesy! It fit her horse REALLY well though, so that's why she went with it. But the leather was poor Argentinian quality and the grain texture wore smooth within months. Also, the stitching seemed kind of "loose" after a while. I also had an HDR Pro (the higher-end line) for a little while. It's low price and pretty looks lured me in. The flaps wore horribly and had very dark marks from the stirrup leathers. I don't think they used a very good tanning method for their leather. Overall, it also felt kind of like cardboard.
If you ride a variety of horses, you may consider one with a changeable gullet. Although, I'm not sure who else makes them besides Wintec (blech!) and Bates ($900+ used). Or, get a wool/sheepskin pad (like Beval's) to help your saddle be more comfortable for multiple horses.
As for the Dover...I haven't had any personal experience with them. But, I know at one point, they went by the name of "Trainers" and these were designed/produced by Ruiz Diaz who now makes the new (Argentina) Pessoas.
Overall, I would actually recommend buying a used, quality saddle over a new, mass-produced saddle. Especially if the horse may spook and drop or scratch your saddle. Or maybe in a year you'll get your own horse who requires a wide tree and you'll have to replace your saddle! Ya never know. =) Of course, it's easier to tell your parents, "Get the 17", Medium Tree, HDR Pro Close Contact down at Bob's Tack Shop" than "surf eBay for weeks until the perfect saddle is listed." Do your local tack shops have consignment/used saddles?
Oh! Have you considered a Beval? They make some awesome saddles and their Stamford is produced with Argentinian leather, so it's less expensive and still really nice! I don't recommend the Princeton though...too cheapo.
Sorry for the long post...I'm very passionate about not riding in crappy saddles!