Familiarize yourself with the costs of materials such as types of flooring, water heaters, heat and a/c units, appliances, counter tops, windows etc.

You're lucky you didn't have a boiler & hydronic heat.

However, the market it seems has heated up EVERYWHERE and I need to recover some funds and this project had so much meat on the bone it will give me the ability once sold to payoff my own house. 

Window Kit 112-in x 84-in Clear Heat-control Window Film https://www.lowes.com/pd/3M-Wi...If that doesn’t work, you could try lowering interior humidity (unlikely to make significant difference) or upgrading the windows. 

As for heat, add baseboard heaters.

The house is a 3bd 2.5ba with a ADU with a bonus room (with its own cooling and heating system).

If this doesn't do it, I would say the odds are a) it's defective, b) they are trying to get your goat, or c) they really need a regular flow shower head.That's a good idea, I didn't think about checking the heat setting.

The property:Is being offered for $80k~2500 square/ftIs a double lot3 bedroom, 1 bathroom, living room, kitchen, office.Uses steam heat, with a new furnace being installed now.

Hired out electrical plumbing and heating.

You can put pipe insulation on them and apparently they sell heat tape as well.