Thanks for all the help. I finally decided/purchased/installed my Pentair 2-speed 1.5 hp Whisperflo 011522 and C&C plus 425 sq ft cartridge filter this past weekend.
Pump: The wiring was clear and simple and the 2-speed worked instantly. On low speed, it's very quiet but enough flow to operate my KK and the Aquarite. When I first turned it on without the KK, the heater's "normal" indicator light was on too so I was so happy but later, I think after hooking up the KK, it turned off. I know the KK can restrict the flow so may be it's the cause but I am not sure until I do more tests. It definitely won't work with spa. On hight speed, it's loud, almost like my 30 yrs 2hp motor. But it could be the the noise from the water flow because even with 0.5 hp less, this pump seems to move much more water.
Filter: With 0.5 less hp than the old pump and much more sq ft of filtering (old Purex of 48 sq ft), setting at low speed, my filter's pressure reading was zero (it did increase from below zero to zero). It's the good thing I guess. The union fittings helped with the installtion of the pipes. Now, the fastener ring and the clamping system, unless I missed something, is not a very good design. Complared to my 30 yrs old Purex, this clamp system needs a tool (wrench) to remove and you have to remove the nut all the way out in order to take the ring out. And when you fasten it, the heavy duty spring coil rub against the ring and make loud metal-metal sound. Why can't have the same design as the Purex's. The bolt has a plastic knob at the end to turn without the need for a tool. And the other end of the bolt is T shaped that you can just loosen up a few turns and latch on/off to the the end of the ring to fasten/loosing up the ring. I also think the Pentair ring is over kill, way too big and heavy. The instruction says to tap it while fastening but I think I need a third hand to do that. The ring will not rest on the tank but keep slipping down unless it's tight. May be with more experience it will get better. The drain started leakinng a bit when I first turned on the pump but stopped eventually.
Shedule 80 nipple and Plasto-Joint Stik : They don' work very well. I cut one into 2 pieces and used each for the in/out of the pump. I put plenty of the Plasto on the threads based on the instruction and reviews and it still leaked on the pressure side untill I turned it really really tight. The Plasto I think is too dense and makes it harder to turn and from my experience, when you connect threaded pvc pipes, if you keep turning when it's tight, it may crack . I can't tell on the suction side if it leaks or not. The nipple's thread ends seemed to be tapered at the tip, I guess for easy installation but may not seal well at that section.
The hardest part of the job is connecting the pipes because I wanted to make if perfect, no pressure on any of the joints for long term durability, specially at the filter's/heater's union connections. I also used unions at the inlet/outlet of the pumps for easy disconnection in case the pump needs repair/replacement later.
I am happy with the result and again thank you for all the info I've been getting. Can't wait to see my electric bill next month.
Update: It was the Plasto that caused the leaking problem. I took apart the nipple and put 2 layers of teflon tape and top with pipe dope. It went back in smoothly all the way and no more leaks.