Answer your questions:
1: It cannot be connected in parallel, because the old one will no longer be usable. It is difficult to install in parallel, and it will also affect the use of the new one. solar energy.
2: The price of 3500 is high, it should be around 2500. Find a few more brands (never use other brands!)
3: When installation: be careful to check the manufacturer and office, agent telephone number, keep the receipt and monitor it carefully!
4: Recycling old water heaters can be difficult! If it were sold, it would be hard for anyone to want it!