I’m looking for a service that tenants can pay their rent online / auto payments.

This is sure to happen.What is needed is a quality white label service at a decent price point that is coupled with lead generation and legal support. 

Given that fact I am building a duplex, only one potable water meter will be servicing the property.

You must use the services of a qualified intermediary to handle the exchange and they must be in place prior to the closing of the sale. 

You can get service the same day by using what is called "nail and mail service." 

I've used a flat fee service but prefer to use a Realtor that charges 1% instead. 

I use Linnemann Realty for SFH for more than a year and very happy with their services.

Exception, on larger parks/nicer parks, you can afford to maybe higher out the property management services, it is becoming ever more popular, but it is still a long way off from the apartment world.

They're pretty available, can probably pick a blank up from a big apartment complex and customize to suit your needs. 

Also, if I can be of any service on existing properties I'd be happy to take a look for you.