Base filter AND/OR filter grills or is just a base filter enough? Can't tell if I'm being sold snake oil.

Hi all,

I'm very much a noob at the ins and outs of home repair and am not having much luck resolving a disagreement between bids I'm collecting by googling. TLDR in the title, basically is it worth it to install filter grills in lieu of a filter at the base of an intake, is it best to do both or is that too much strain to pull in air, or is it better to just have the one filter at the base and not do anything? More below:

We have a set up in a closet where the coil is installed sitting on top of the furnace on a platform about knee high. This means to remove the coil, the furnace will have to be pulled out first. Replacing the coil is the main job we're looking at doing.

I was advised by an HVAC man that cutting in filter grills at the two closet return grates under my furnace blower at the same time was a good choice. He says this will extend the life of the nine year old blower when combined with cleaning the blower assembly while he's also replacing the coil.

His logic is that with this done, the furnace will last another 10 years along with the new coil and we can replace both at once at the end of the new coil's projected 10 year life span. This is accomplished by not having the filter right up against the intake, which gives it more room to "breath", which I'm guessing means the motor doesn't have to work as hard to suck air through a filter right up against it. I apologize if I'm not making sense or misremembering.

Currently, it's just one filter snug at the base of the furnace. The unit is installed in a closet at an interior corner of the house overlooking stairs, so the two filter grills will form half of a box underneath it (already has holes cut out about knee height, bout 20x16, no filters, just slotted metal grates), solid wall on the other two sides. It's the only unit in a two story 2500 sq ft house in central Texas.

Confusion is, the other two bids I collected both thought I was being sold $450 magic beans and that just the one base filter is fine. Both mentioned being very nervous about not having something to protect immediately in front of the intake in case particles got past the floor filter grates.

Of course, all three had different recommendations about what to do about the furnace. One thinks we should clean and reinstall, one thinks we should just replace it when we replace the coil, one thinks they'll just leave it alone cause it's fine. All three are well rated companies, so I'm just not sure who to listen to.

I'm hoping y'all can at least point me to were I can research further if not make a suggestion. Thanks for the read!

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