I need to replace a decaying old DE filter that is probably undersized(see signature for pool details). I'm considering either Sand or DE (familiar with what goes into DE) and believe that I should replace all of the PVC Piping pictured as well. Plan to switch to a MPV at the same time. I plan to clean the 'pad' and build an enclosure. Previous owners left it to the elements. Tried to use search and the pool school, just a few questions before I pull the trigger:
1. Hayward W3S310S/Pentair TR100 30" Sand filters and the Pentair Quad DE 60 are comparable costs, am I understanding the conversion correctly that the Quad DE 60 would be the most 'oversized' option for my setup? For sand it seems that I should be over-sizing with a TR140 but the price jump/availability seems significant.
2. I need to replicate the check valve between the pump and the filter, do I need another check valve anywhere else?
3. Looks like the previous setup has a cutoff for returns 1/2. I've never used it but is there an advantage to keeping that setup that I may not be considering?
4. For the outlet from the pump it looks like the 90 comes right out of the top, I'm hoping that's just threaded in and after cutting I can just remove it. I replaced the motor in 2020 and plan to run it into the ground before upgrading the pump. If that's glued in I'll have to replace the housing or new pump?
5. Anything I'm not considering?

Thanks for your time.
1. Hayward W3S310S/Pentair TR100 30" Sand filters and the Pentair Quad DE 60 are comparable costs, am I understanding the conversion correctly that the Quad DE 60 would be the most 'oversized' option for my setup? For sand it seems that I should be over-sizing with a TR140 but the price jump/availability seems significant.
2. I need to replicate the check valve between the pump and the filter, do I need another check valve anywhere else?
3. Looks like the previous setup has a cutoff for returns 1/2. I've never used it but is there an advantage to keeping that setup that I may not be considering?
4. For the outlet from the pump it looks like the 90 comes right out of the top, I'm hoping that's just threaded in and after cutting I can just remove it. I replaced the motor in 2020 and plan to run it into the ground before upgrading the pump. If that's glued in I'll have to replace the housing or new pump?
5. Anything I'm not considering?

Thanks for your time.