Sizing HVAC system before and after addtion

[python01]

I have 1100sqft home (measured on the outside). It is single level with full basement (each 1100sqft). I know the basement is not taken into account when sizing heating system but it is used as separate 4 bedroom rented apartment so maybe I should add the footage?
Anyways, I would like to get new HVAC system for the house since now there is no A/C and furnace is probably about 30 years old. However I am planning to add another floor to the house in next 2 years and if I get new complete HVAC system I wish it could accommodate the the addition. Is there any way I can do it and what size of HVAC system I would need before and after the addition?
Through some research I have done so far I have read that I could use two separate heating systems or do a bay-pass if I decide to go with larger system. Though either way is possible solution I wonder why I can not just get bigger system now (like 2 tones)? One place told me the system could brake if it is too big. I understand that oversized A/C will not perform very well but maybe somebody can shine some light why oversized HVAC system could brake?

 

[nealtw]

Just some thoughts for you that likely won't answer any of your questions. Make sure you plan ahead for where the duct work will run to the addition. Air conditioners work hard to lower temp in the house by 20 degrees and the basement is most often 20 degrees lower than the house, don't waist time and money on the basement. Where ever possible you want to seperate a suite with a fire break which is defeated with a common furnace. I would look for the size of furnace for the upstairs with the addition. When you do the addition seperate the basement with it's own funace. Fire stopped and seperate controls.