Yeah, I'd guess those 18Sound cabs are better than your current 2x15. So if you have both of those, you could sell your 2x15 and have a much easier rig to deal with.
Did you just want a single 15 to put by the drummer? Isn't that kind of his responsibility? And can he really not hear you playing through a 2x15? That's kind of weird. If you still needed a 3rd 15 for the drummer, you could pick up the Peavey if it's still available, or find something else similar. But what were you going to run the 3rd 15 from anyway? Most amps won't be too fond of the 2.67 ohms you'll get from 3 8 ohm cabs.
Oh, and definitely just buy speakon cables to use with the 18Sound cabs. If you're lazy, that's easier anyway, than trying to mess with changing the jacks.
No, no. I don't have a car, so it's all about space when we drive to our gig, and the 2x15 is a lot bigger than these little cabs. So that's why I needed a smaller cab just to put it in his house, so my bigger rig can go with the other band. And if I buy this last cab, I've also got a small one at home!
(Yeah, I'm not getting rid of my 2x15 anytime soon... it's a match with my tube head and it sounds amazing together, so that's my keeper for a bigger rig!)