if you got a dm6 your gonna need a fast hopper to keep up once you start to get good at walking it.
a loader is just a generic name for a hopper, and a "rotor" is just what Dye markets their newest loader/hopper as.
i have just upgraded to a rotor but have yet to use it in a game so i cant comment on its performance, but the reasons i went for the rotor over the halo or velocity's are A> they have finally made a hopper look cool and B> i saw a vid on youtube where the dye rep did everything he could to trash the rotor,he threw it around, he submerged completely in water and it still worked underwater, the electronics were not affected at all.
it may just have been a gimmick but it worked on me
what can i say.
of course there are other less expensive hoppers out there that will do just as good a job, it all depends on your personal taste and budget, rotors are near the top of the price list afterall.
as for air tanks, obviously you wont go for Co2, so again it just depends on your budget i suppose,there are a number of different retailers in the u.k that sell tanks or you could even look on the classifieds section on here.
the different psi/cu just represents the capacity of the tank and how much air it can hold,alot of fields only fill to 3000 so anything bigger may be a waste unless you play a big scenario game.
look on youtube under "TechPb" vids,Mike Phillips the guy who created it does alot of videos explaining all the ins and outs of hoppers and tanks and masks etc etc especially for beginners
good luck