Hello shadi, welcome to the forum.
I can't answer all your questions, but I will tell you about my experience with my '03.
First off, I think the quality of my Avalon is right up there with the finest of Toyotas' cars. I heard there were some problems with the previous generations HVAC systems, but if that's the case, they certainly worked them out with the update. I live in the southwest and the AC is necessary and is the best I've ever had. I like the heated seats in my XLS too.
The car is comfortable and well suited to the city driving I do.
Under warranty, I had to replace a dead battery and have the front brakes repaired for a bad shimmy at highway speeds. They simply turned the rotors, and maybe fixed a stuck pad which was the cause of the problem.
At approx. 30K miles I replaced the struts and threw on new tires. I was surprised the struts wore out so soon, but have come to believe this is common with oem equipment.
With routine maintenance, nothing else has gone wrong with my car. And, in fact, since the strut and tire replacement it is riding and handling better than new.
I'll leave you to determine the value of used Avalons, but the bottom line is, in my opinion, with a well maintained vehicle with reasonable mileage, you won't go wrong buying one.
Good luck. See you on the forums.