Remember, HEPA filters like the one I described will take their OWN 1 HP fan to push the air thru them at the required rate. Your furnace fan has not a prayer doing this. $220 for the filter, several hundred more for the second fan, ??? for the install ??
Yes, a MERV 7 1" or 2"in front of this catches most of the junk and lets the HEPA filter last longer,but you MUST remember to change the pre filter regularly, perhaps more often than than you change your existing filter.
A step down from this is NOT a shallower HEPA filter. A 6 inch deep version will NOT provide nearly the same level of filtering or the flow required.
The step down is to get a MERV 12 Purolator Defiant Extended Surface bag filter. Change this annually as a part of the HVAC inspection. These
24x24x19 6 pocket filters have 42 sq ft of 0.25 thick filter media. They will capture 75-80% of the 0.3 micron stuff (vs 99.97% for HEPA). $28 for the filter, Roughly $100 for the mounting hardware, ?? for the install ??