How do I know the installation price is ok?
Get three similar bids. If the bid is not the same in every detail, then the comparison is impossible for the homeowner. If the different bids specify different sized equipment, then you either need an engineer to size the equipment, or get a bond against incorrect work. Remember, it aint the smooth talk that builds the best system. Many contractors do not want to explain their reasoning, because the homeowner is normally not educated enough to understand. At the same time many contractors think they are right and do not want second opinions, when in fact they only guess in many cases.