I have hade some experience with that type of hydronic system, You are right in your consideration of slowing the air down. The return air on most furnaces are sized about 10x25 and putting a coil in that small space would slow things down a bit. Secondly the filtration system is usually right where the R/A drop hooks up to the furnace so if you put the coil upstream from that it will clog up in short order. The best approach is to put a hydronic coil above the A coil in the plenum and set it up on a 2 stage temp. controller so the solar and furnace do not run at the same time. you would also require zone valves to direct the hot water from the panels to the water heater and to the heating coil on a priorty basis, with hot water first then space heating after that has reached temperature. first determine how long it takes to make the hot water, if it takes most of the day you are not going to benifit from the expense to do all of this unless you add the appropriate no. of panels. another consideration is the blower size. There is a lot more I need to know in order to design a system, furn make, model, or gas, elec., your location and solar zone, etc. e.mail me at oltoby58 @ yahoo.com
and we can chat about this.