What filter to buy

[CarlD]

You installed the spider gasket IN the multiport valve. Unless your filter's tank is damaged, even replacing the entire multivalve is far cheaper than a whole new filter. BD is right: That pool store isn't to be trusted.
The only reason to replace a sand filter entirely is if the tank is damaged or it's too small for your pool or it's a model for which parts are no longer available. Otherwise, you can replace the insides, the multivalve, even the drain cap, without ever changing the main tank.

[BigDave]

Wow! I just re-read my post from yesterday. I think my phone thinks it's smarter than me - probably is.
I meant to say: "... valve rotor was installed in the wrong ...". Yikes! Sorry.