List of manufactueres here:
http://www.builditsolar.com/References/Suppliers/SolarHeating.htm#FlatPlate

The SRCC certification means they have passed some punishment tests, and its also necessary if you want to get the tax rebate.

The King Solar page you referenced was for a domestic water heating system.  Space heating systems are similar, but with some differences.  I'd take a look here:
http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm
in the "Active" section to get more info on space heating systems.  There are some examples of the kind of system you want to do there.

Baseboard heaters are not the best match for solar heating as they take pretty high temperature water to get to their rated output, and solar collectors get less efficient as you try to heat the water to higher temps (the losses go up).
Radiant floor heating is the best match as it can use water all the way down to 85F or so.

I've got both solar air heating collectors and a solar water heating collectors in a system I use for space heating.  My feeling is that they both work well, but the water heating systems are several times more complex, more maintenance prone, and more expensive.  On the other hand, the water heating systems can store heat, which is difficult with air systems.  But, you might consider whether you can add some more solar air heating collectors and (perhaps) some more thermal mass.  With more thermal mass, the room will heat more slowly and carry more heat into the evening.  This might work well for an office space that does not need to be heated all night.

Your pal