We are looking into adding a 2nd bathroom, some electric work and plumbing (because of 2nd bathroom) and cosmetic work.

For example: you can decide which plumber to send to fix the leak, pay the plumbing bill, pay property taxes, shop for insurance, etc.

Then add some automation software to help reduce the work required for guest communication.When you get something electrical, plumbing, or appliance related, head over to Angies List or HomeAdvisor or one of those companies and do a search for a quality tradesperson.We run 2 short-term rentals remotely using the above. 

I Went to school for hvac/plumbing in highschool and ive been doing sidework painting for about 5 years.The house was laid out perfectly to add the bathroom just filled some empty space in the laundry room with it.

@Logan Caudill You'll probably have to figure in the acquisition costs to purchase the mobile homes, the fix up involved, and the moving costs (including hookups such as water, electrical, plumbing, etc).

Like electric, plumbing and etc?

.- Complete bathroom, kitchen updateI would like to have a deep understanding of the things to look out for, the considerations to account for such as for example the plumbing pipes, the ductwork, the electrical upgrades, weighing up the different options, envisioning the space.

Some HOA's cover almost nothing and others cover everything to do with exterior maintenance, plumbing and electrical (in the walls only).

If I were you, I would make a call to your local county were you would get your permit and ask them about electrical and plumbing before you do anything. 

Items such as the heating and air conditioning, roofing, siding, hot water heater, plumbing and flooring issues are all things to keep in mind.