one contractors recommending belowClean blower wheel inside furnace around 345$ ( he says cutting air flow efficiency by 20%)To replace blower motor capacitor around 125$ (says reading should be 5.0 but it is 3.9 may fail any time)To replace condense capacity 135$(says reading should be 5.0/40 but it is 4.8/37  may fail any time)What are the worst numbers until which these capacitors work before worrying about replacing them?

If possible, use the clear plastic flaps to curtail air flow.  

The wall gets cold from the exterior and if something is blocking airflow on the wall, it stays cold.

Do you think it's better to use the cheaper filters or the kind that say they offer 90 days of air flow?

Some filters have bold arrows showing the direction of air flow.

If the temperature drop is less than 25-30 degrees, there might be a problem and calling a contractor is warranted.I also check for air flow just using my hand.

Sounds like the biggest price you're paying is adding air flow into the attic with the baffles.

On those direct vented high efficiency systems, the air intake pipe can be blocked by trash flying around outdoors on a windy day; the air intake sucks outdoor air, and those plastic grocery bags flying around are ideal for blocking that air flow. 

When you "enclose" these appliances, they have some spacing requirements for air flow around the sides (cooling / safety), usually in the installation manual or on the units themselves in some cases.

Filter had been changed a few weeks earlier and was clean enough but supposedly restricted air flow can cause the problem...guy said to wait 4 hours and turn system back on if it happened again.