Anybody know the rules for replacing water heaters these days?

There are many, many other reasons and examples of how the business model you are thinking of could easily go eschew too ...Much better off staying local and self managing ... or staying local with a PM that you can closely oversee to validate that they are taking care of your business, and if not much easier to fire, take over in the interim, and then replace them.

You can also get in touch with several reputable contractors in your area that specialize in certain things and get some reasonable ballpark figures - electricians for panel replacements, roofers for asphalt shingle roof replacement per square, etc.

Specifically, I would like to know: 1) is my assessment of the code accurate; 2) should I replace and hardwire the smoke alarms; 3) am I required to disclose code violations to the renewing (or existing) tenants; and 4) can you recommend someone to do the work?

The house need to replace roof, siding, and outside stair way have some damage.

According to the owner, the roof was replaced in 2009.

Per seller house needs new carpet, paint, landscaping, and tile replaced in one bathroom (15-20K?). 

However, I think I'd have to replace it for it to sell anyways, so its already a cost factored in whether I continue to rent it out or sell it. 

Any drawbacks to knocking down the garage and NOT replacing it for a rental duplex?

If you know your roof is old, consider replacing the entire thing.