I'm having trouble figuring out what kind of filter I can use for my furnace. I've read a little bit about it and it seems that a residential furnace might not be able to handle the HEPA filter due to the higher airflow resistance. Even MERV 13 might already be too high.
The MERV or other rating of a filter is not what's important to your furnace (it's what's important to the humans) but what matters is how much airflow through the filter can be achieved by the furnace fan and how much pressure drop the filter induces.
If you get a MERV 12 filter with LOTS of filter material, it may flow more air at a lower pressure drop than a MERV 6 filter with minimal filter material. Look for more pleats in the filter or modify (or hire an HVAC contractor to modify) your furnace filter cabinet to accept a thicker filter cartridge.
For reference, in my furnace during an HVAC consultation, we saw the same 0.5 inches of water pressure drop from a cheap 3M Filtrete MERV 7 filter as with a K&N reusable MERV 12 filter. The K&N has MUCH more filter material.
You don't want to restrict the airflow to your furnace any more than is necessary. Most furnaces will have their required maximum pressure drop from the duct work and filter printed on a label inside. You can get a differential pressure meter to measure the actual pressure drops yourself, or hire an HVAC technician to do an audit of your furnace (often they'll do lots of other neat tests and give you lots of data and reports and info, too).
This is probably the most important. If you work from home, it might be prudent to at least have a one-room HEPA filter in your home office.