They don't do anything that you can't do yourself. If there's an inaccurate record, just send them a letter informing them that you want it investigated according to the fair debt collections act, otherwise it needs to be removed within 30 days.
This. You can do it all online too. Just start a dispute and in a few months or sooner they'll remove it and you're good.
I dont agree with OP about not using credit. Never use your own cash. Interest rates for car and mortgages are not that high for good credit folks.
On certified cars, I get 0.9-1.9% apr which is nothing. That 50% down on the next house is better served invested...with your history, you can get a 3-4% loan on a house which in hindsight is nothing.