Be carefull if you use Home Depot (and probably Lowe's) for installation; there have been some real horror stories.
You might also want to check with your utility company. Some will "rent" a water heater to you for low $$$ fee that includes repairs if anthing goes wrong.
As for something new leaking after a plumber's visit, it may be the plumber but it could also be repairs/changes made in one area increase water flow/pressure that blows out older connections. Just a thought.
Best of luck with this! My house is 1962 and I'm always wondering what will go next.