The pole barns I speak off were always wooden posts. Usually cedar. But nothing that is wood or steel that goes into the ground will last forever. Sooner or later I would expect corrosion to eat through the metal.
Are you perhaps talking about something a bit different? A metal building that was built on concrete forms with the steel bolted on top? That kind of setup seems very solid. I have seen the bottom of steel buildings show rot after 20-40 years. Maybe it has to do more with water coming from the roof and splash. I once saw a wooden house built on a slab (about 20-30 years old) with the same problems. Maybe proper site work would prevent it. Maybe not. I would expect that it would be possible to remove the outside skin when it happens just like you would replace vinyl siding. It seems if a building is built at least a foot . maybe 18-24" above the ground level splash is not so much of a problem as it is when it is within 2 or 3 inches.
Congratulations on being smart enough to save your money and be able to buy for cash. The best answer for you about financing is easy for you to discover. Talk to 2 or 3 local banks. Talk to their mortgage person and ask them if they would finance this kind of house. Take pictures with you. If they do not agree to finance it realize you may never be able to sell if you ever wanted to.
Even if you were 20 years old now and wanted to live there forever think about when you turn 65 and want to move to florida or arizona for health reasons. Plans sometimes change. Be sure you buy something that will allow your plans to change if they need to.
If you were thinking about building from scratch look at a quad lock it is insulated foam panels you pour concrete between. Very energy efficient. Slightly expensive to build (they say 5% more) but will save you a lot of money in the long term. It is said to cost about the same per month when financed including heat and taxes as standard wood houses. You could put any siding on you want. Being concrete and foam the house shell will never rot. It is also said to be very quiet compared to normal construction. The houses look like normal houses except for having slightly thicker walls.
If I was to build new I would definately consider that system. There are other competing manufacturers of foam and concrete houses. Consider them all.
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