On the cabin filters, you can save alot on them if you go to Home DePot
and buy their cut to size (blue in color) dense air filter . I think it
comes 20x25"  and is cheap yet effective.

Cant help you on the evaporator replacement but if it were me, i wouldnt
purchase a replacement evaporator until i could see if i could gain
access to the evaporator to determine where the leak is , and if
possible,  fix the leak with special Evaporator Epoxee . Ive used it
before and it works well .  If the leak wasnt accessible, then id put a
new evaporator in while the dash is disassembled.  By the way...once you
have it installed,  are you familiar and qaulified to pull a deep
vaccuum on the system and recharge it exactly to the manufacturers specs
?

You'll need to remove the glove box and housing and, of course, discharge
the A/C.  Disconnect the lines, and pretty much unbolt the evaporator and
wrestle it out.  Use reasonable judgment regarding whether anything else
must be loosened or moved to free up the evaporator.