The original post is rather confusing.
"small rust was on the jets" makes no sense. What 'jets'? The coolant system does not employ jets to squirt coolant anywhere.
It is usual for the cooling system to show some signs of rust, but not more than a 'light dusting'. There should be no loose particles.
Your local mechanic saying that rust in the cooling system was a sign that water was getting in, has either lost a lot in translation, or you need to keep well away from him. Coolant is a mixture of water and the coolant, which is a blend of chemicals. So water is part of this, so I would hope there was water in it.
Being a 2008 vehicle, this should use OAT type coolant. Normal change interval for this type is 5 years. Mileage is not particularly relevant. A 5000km change interval is only correct if the vehicle only travels 1000km per year.
The coolant does several jobs.
1. The most important is a corrosion inhibitor. This is what wears out, and why the coolant needs to be changed. As the inhibitors lose efficacy, corrosion can occur.
Some cylinder blocks are cast iron, and will rust, but slowly. The core plugs, that close off the holes in the block where the casting sand was removed after casting, will rust quickly once the inhibitors lose efficacy. When one of these lets go, all the coolant escapes, and unless stopped very quickly, you'd destroy the engine.
Cylinder heads are mostly aluminium, which will dissolve very quickly in water, so corrosion inhibitors are critical.
2. The coolant also raises the boiling point of the liquid. That along with pressurisation of the system as the temperature rises, allows temperatures above the normal boiling point of water, resulting in more efficient combustion. This is not affected much as the coolant ages.
3. Anti-freeze. The coolant also lowers the freezing point of the liquid, well below that of plain water. This is important in areas where the temperature drops below freezing, as water expands as it freezes, and will crack cylinder blocks. Many areas do not encounter temperatures below freezing, and owners consider not using coolant. That destroys engines quite quickly. This is why it is referred to as coolant, not anti-freeze.
Changing the coolant is messy, but not difficult. Remove the bottom hose from the radiator, and it will all flood out. Some may remain, so it is then recommended that the system be flushed with plain water, drained again, before refilling with the correct coolant mixture, usually 50/50 coolant to water. Anyone telling you this is a difficult job is not to be trusted with your vehicle. Ideally, the drained coolant should be caught, and disposed of in an environmentally friendly manner. Garages will usually have a drum, that gets collected and reprocessed. In the UK, local household refuse centres usually have a tank for it. Catching it is often difficult. Spillage should be washed away with a lot of water to dilute it as much as possible. If the domestic or street drain goes to the local sewage works, a little coolant will do no harm. If the coolant escapes to water courses or into the ground, that is bad news. As it decomposes, it has a high oxygen demand, so starves everything around it of oxygen. In rivers, everything dies, fish, plantlife, etc., until the coolant is fully decomposed. Soaked into the ground, it has a similar effect.
Find a garage who treats this as a simple job, without drama. Should not be more than an hour's work, plus the new coolant cost.
