Services should be conducted annually
After work is completed and the equipment is registered, manufacturing companies give 10yrs of part warranty.
There is no right or wrong answer to what equipment is right in most cases. It's all about preference and what better fits once a budget as cost to different systems vary in cost.
Manufacturing companies would say that after 10yrs equipment's start to loss their efficiency. I'll say that after the 10yrs; if your equipment begins to give you major issues it should be replaced to avoid the inconveniences that breakdowns can cause and cost.
Air filters should be checked every three months and if necessary, replaced. Again, this is another question that can vary different with each home. Home with hardwood floors would tend to have to replace earlier than homes with rugs. Home with many animals with carpets will also tend to have to change earlier. I'll say to be safe; check it every month and only if it's needed you change. And only use Merv 8 filters not those cheap ones
air ducts should be cleaned every 5-7yrs dependent on the situation
Anywhere from 35-45% humidity depending on the outdoor temperature. Temperatures under 30 degrees 35% and over 40-45 degrees 45%
Humidifiers are good to have in homes when there's people who experience with dry skin, allergies and Asthma.
make sure the equipment is sized properly and the duct work are also and heating and cooling a home shouldn't be an issue
what type of equipment should one get all depends on their preference and financial ability