Hi Hari
Hearty Congrats for buying an Amby. Its rare to get a used Amby in good condition now a days (even though there are many tourist cars come for sale). Your car looks so pretty which shows your effort you have taken for it. Regarding overheating, radiator leak should be attended immediately, because it literally takes out the coolant in the radiator slowly but steadily. Me too was facing the same problem with my 1984 Amby with 4FB1 Isuzu engine.
Regarding the temperature at half mark, no need to worry about that as its a standard Amby stuff. Regarding waterwash, my sincere advise is please avoid water wash for ur car in service stations, because most of the service stations use hard water, which collects at the running board & doors. The service guys wont wipe the water collected underside which leads to rust. so better wash the car by yourself.
During monsoons its better to apply anti rust coats which will last min 6 months. If you live in coastal area, then its a must to wash the car once in a month.
Happy riding...