LD, I do reload 5.7X28 and have been loading it for about 4 years. I do not own one of the guns you talked about, the gun I shoot it out of, now, is a custom barrel for my Thompson Contender. I have not tried to buy any brass for it because I got such a deal on some ammo a couple of years ago that I have plenty of "fodder" to feed it.
When I first started reloading it, I had both a PS90 and a FiveseveN pistol. The blowback design of both of those weapons caused to neck of the brass to be pushed forward about 1/8 of an inch and rounded the shoulder. When I first started reloading the 5.7, I used an RCBS full length die set (which did not cost $75 at that time). I no longer use that FL die since I now only neck size them, Redding makes a set of neck size dies for it.
I posted a picture of the brass at the time I first started messing with the 5.7X28. Here is a link to that thread from 2007. http://www.thefirearmsforum.com/show...ghlight=5.7X28
I don't shoot a lot of them so I never tried to find a Dillon conversion kit for it (and now I won't since you checked on it). I load it on my LEE single stage press use the Reddign Neck die and the RCBS bullet seating/crimp die.
EDIT: I did find these at MidwayUSA:
Full Length Die Set: http://www.midwayusa.com/product/816...set-57x28mm-fn
Small Base Die Set: http://www.midwayusa.com/product/174...set-57x28mm-fn
If you do buy the gun in 5.7 and want to start reloadind for it, I will be happy to send you my RCBS FL die but you will have to locate a bullet seat die for it. I know it uses the same Lyman and Hornady shell holder as the .25 Automatic (RCBS and Redding Shell holders are not the same as the .25)