The more liabilities it has the more the HOA should/will collect such as roofing, pools, maintenance of roads, parking, gym, etc. 

Roofs 25 year life span, floors 10 year life span. appliances and hot water heater 12 year life span, hvac 20 year life span, bath and kitchen remodel 20 year life span, etc.

. $6000/10 year life span/12 months in a year= $50 per month for flooring capex.You won't be able to replace the roof 25 year life span, appliances and hot water heater 12 year life span, hvac 20 year life span, bath and kitchen remodel 20 year life span.

2. main bathroom: the toilet wasn't flushing and will most likely need to be replaced. the tiles on the floor and along the wall were also showing signs of coming off and will need to be replaced. 3. flat roofing material has aged and will most likely need to be replaced soon. 4. water heater is working but has met its life expectancy; will need to be replaced soon.5. water supply piping has started rusting in certain areas and will soon need to be replaced6. radiator in the main bathroom on 2nd floor not functioning7. oil/water boiler has met life expectancy and will need to be replaced soon8. chimney shows major deterioration/damage from exterior and needs further inspection9. wood framing in the rear of basement shows evidence of rot. the kitchen is directly above this area of the basement. extent of damage could not be determined due to insulations and will need further inspection10. pavement in the back of the property have cracks that are leading rain water towards the house. we were hoping to renovate this so that the water is diverted away from the house.

Also how old is the roof?

It don't believe anything other than the roof and hot water heater has been updated since the 50's.

You only have a portion of the roof, plumbing, etc. 

We just recently did a full re roof and raised the roof to give it more space.

My original plan was to build a 1.5 story garage in the back with an ADU on top and the roof top deck on top of that but that doesn't seem possible with my zoning. 

Should I buy a duplex with all the major expenses(roof,hvac, heater) 20 years old?