questions of reloading brand and interchangeability?

I also prefer the RCBS set up. It's never let me down in 30 years of reloading. All the dies should be threaded the same. Make sure you get dies for a semi auto, not for a bolt action. The main difference is that you resize the full length for semi. Also, you should be aware that you cannot safely use standard primers in a semi auto like the AR15. You will get a slam fire from the light primer. You must , I repeat you MUST use the military style primer from CCI to avoid ruining your rifle and possibly injuring or killing yourself. Get a good reloading manual ( I prefer speer). whenever possible, I buy carbide dies from RCBS to avoid having to lube everything. You will need a shell holder for the cartridge you reload, and I also buy a lee auto primer and shell holder for that. It's much faster than a single stage primer set up like most presses come with. Get with someone who has bee reloading for a while to get you set up and going if possible. Then, read every set of directions twice, go slow, concentrate and be careful. Reloading can be a lot of fun, very relaxing, and at the same time, very dangerous.

Good luck

Ditto Randy's answer. RCBS gear is very good (I own and operate 15 RCBS presses in my Reloading Lab) and the Customer service is top-notch. For extreme accuracy, Redding dies are the best but generally for AR-15's you would not be able to tell the difference in accuracy due to the rifle not being able to shoot groups below about 1 MOA to take advantage of the Redding dies. Stick with one company as a source for new equipment and Thank your girlfriend, she's a keeper!!!

Dies generally are all threaded the same, and most common dies will fit most common presses. I'd stick with the RCBS, since you already have it. I use Lee dies, and things will break every now and again, I've heard the same from their presses, but they are inexpensive to replace. If you can get a set of RCBS dies, I'd go with those first.