I doubt if we have the hardness you recorded but yearly preventative maintenance of any system is imperative to longevity.Given the precarious attic location I would definitely review the obvious advantages of an on-demand system.I have also designed a simple fix for the 'cold water sandwiching' issue that plagues the on-demand systems with feed water temperature fluctuations common in colder climates. 

I've seen to many people try to DIY plumbing and get burned a few weeks later when they forgot to do something like re-wrap the pipes in cold climates...

They are numbered 1-13 in our rental agreement.Here you go:13.Any illegal substance use on the part of TENANT, his/her family or guests shall be cause for expedited eviction.12.No parities may be held at this residence.A party is defined as any gathering of guests that creates a noise level that can be heard by neighboring tenants.11.Absolutely no waterbeds or water furnishings may be used in or on the premises unless otherwise provided herein.10.Nails may not be put in the walls without prior written permission from the LANDLORD.Nail holes should be small and unnoticeable.If the holes are large or visible, a charge of $5.00 for each hole will be made for repair.No decals or contact paper is to be put on any woodwork, walls, appliances, or cabinets.Nothing may be taped to any walls.9.TENANT will see that the conduct of him/her self, his/her family, and guests are not disorderly, boisterous, or unlawful; that it does not interfere with the rights or comforts of the other persons on or adjacent to the premises and that visits by the police to the premises for any of the above is grounds for eviction.8.TENANT will promptly report all leaking faucets or toilets.Water shall not be left running except in winter, when it is sometimes used to prevent frozen pipes.Tenant will accept all responsibility for due to neglect, including frozen pipes. 7.TENANT will properly operate all appliances, fixtures, and plumbing supplied by the LANDLORD.This includes, but is not limited to, coin operated appliances used by TENANTS.6.TENANT will keep clean and sanitary and comply with all laws and ordinances.5.TENANT will reimburse LANDLORD for replacing all broken glass regardless of cause.4.The LANDLORD will maintain the yards in cases of an apartment building with common grounds.TENANT will in case show proper respect and care of the surrounding yard and property of the residence.TENANT agrees to pay for all damages done to the yards, shrubs, trees, etc. by him/ herself, guests or agents.Parking any motor vehicles on the lawn or landscape is also forbidden.

My story sounds similar to yours, except that I did mine in the cold climate, the Winter Park, CO area.

Those will require a little attention for the first year or so but after that they'd probably do OK on their own in your climate. 

Conversely, we have seen our political climate go in the opposite direction, particularly with regard to growth and development” said Sally.

But I can't get myself to just take a bunch of little vacations just to see an area so I've got a proposition for investors in a warm climate..

The climate now is nothing like the climate then. 

The following are some property selection guidelines.Older properties require more maintenance than newer properties.Composition roofs require more maintenance than tile roofs.Properties in climates with hard freezes require more maintenance than properties in mild climates.Properties in locations with a lot of moisture require more maintenance than properties in dryer climates.Wood siding requires more maintenance than aluminum or stucco siding.Properties with lush vegetation require more maintenance than properties with little or no vegetation.Locations with high levels of termite activity will require more maintenance than areas with little or no termite activity.The tenant pool has a significant impact on your ongoing maintenance cost.

Similar if asphalt shingle do not last 25 years in your market due to climate then the cost goes up appropriately.Fortunately, with $400/month projected cash flow you can be off a little on your cap expense estimate and still be fine.good luck