The nitrifying bacteria are still there but in dormant stage, as soon as you start adding fish the bacteria will "wake up" and do their business, just don´t add too many fish too soon, saving water is a waste of time, saving the gravel and filter media is another thing, that´s where the bacteria are. You can rinse the gravel to eliminate the debris that has built over the months, re arrange the decoration, etc and fill up again the tank with new fresh water turn on everything again and wait a coupple of days in order for the tank to settle down, then you can start stocking fish again.

Does your tank not have a lid? If you put something over the top of it, the evaporated water should condense and fall back into the tank.

When you say the glass get "etched," you mean white powdery stuff get dried up on the glass? That is mineral deposit (calcium carbonate, etc.) that can be washed away using vinegar and some scrubbing with plastic pads.

Six months of fish absence should be long enough for any disease-causing organisms to die out, if there were any before.

Like Raul in mexico said, the good bacteria mainly grow in filter media and surfaces of tank gravel. I would lightly wash the filter media and the gravel with tank water or rain water, NOT straight tap water.

It's important to add fishes slowly, over weeks, for the bacteria population to grow with the fish population.

What species of fish are you getting? Many cichlids and livebearers love hard water!

A good glass canopy/lid by Perfecto or All Glass Aquarium will keep evaporation and heat loss to a minimum. The plastic ones that are a hood light combo are not the best for your problem. IMO a fishless cycle is best. This is done by adding ammonia to the water and testing to make sure that you are turning the ammonia into the end product nitrate. Durring this no ammonia or nitrite should be detectable within 24hrs after adding a dose of ammonia. This way your tank is fully cycled before subjecting fish to toxic water. Some people do this to be "humane". I do it because it is quicker and a less PITA in the long run. I wouldn't count on the bacteria being there at all after 6 months without ammonia supplement. If no fish where inthere or no fish food was added periodically to let rot and produce ammonia, I would bet the farm that the beneficial bacteria is gone. Read the first 5 articles in the link below. At least the first and 3rd. If you want to have a clean start and do it right don't count on "maybes". Just my opinion, good luck.

Here is a link that might be useful: Lyretails aquarium bio-chemistry 101