The house is roughly 3600 sf. 90% of the house is forced hotair. A finished inlaw apartment and a family room off the back of the house are hotwater baseboards. Currently, there is an old boiler and a forced hot air furnace in place and I am looking for advice to replace the current system.
I would keep the systems split as they are unless you are considering central air for the section of house that has the boiler system. What type of fuel do you use? Definately upgrade your systems if you are using gas fuel. You can get a 95% eff. boiler that will also heat your domestic hot water. There haven't been to many changes with oil as far as eff. rating. The most I've seen is 82% eff.
I have very little experience with a hydro air system, not sure how that will save you any money.
As far as cost, it really depends on effiency and equip. you purchase and the quality of your contractor. There are so many variables involved. In my area of Ohio, to put in a 95% eff. gas boiler and 95% gas furnace and 15 seer ac it would probably cost between $18- 25,000 depnding on what changes that had to be made and what eqip. puchased. It really depends on cost of living in the area you live. If you are doing it yourself or hiring it done. You can purchase equip. for less than $10,000 dollars, but to have it installed by a reputable contracter or take your chances with you doing it yourself thats your call.