I have to replace the AC unit because the builder used a CHEAPO brand, but I got the seller to reduce the price by the cost of the AC Unit. 

I have been burnt in the past and would like to invest with more confidence and eventually have it be able to replace my salary income, so I can become financially independent.I love real estate and am aiming to make win-win situations.

Would it be most efficient to replace with forced air furnace and air conditioner?  

Age and efficiency of the boiler are big factors - My building has an older boiler that I plan to replace before next year.As others have said, there are a lot of variables and the only way to tell is to get actual records from the buildings you're interested in.

Are there any repairs needed, appliances requiring replacement, etc.?

I do understand that one of those may need to be replaced at some point in time, but they are new right now. 

Take my advice and don't do that. with help you can replace 10 hours of reading/research with one 5 min phone call to a friend. 

This could be anything from a "weeping wall" due to poor grading to a full foundation wall replacement.

I plan on keeping my day job for now, but working hard to replace it.

I replaced them with Faux Wood Blinds from budget blinds.