Stan,
200 CaCO3 mg/l is about 14degrees on the actual Clark scale but there is always a conversion to CaCO3 mg/l hence the 'above 200'.
Unless you need to pressurise the system you will not need the rear frame - not sure they show it for that model in their blurb, I think they show a stand alone vessel & valves.
The offset flue kit allows for a boiler replacement with a conventional center flue hole, & also avoids the flow & return pipe. You can use a straight out flue but you will struggle with the pipe tails & have to shorten one of them, also part of the joint would be in the wall & you would need to do some artistic finishing with plaster to keep it in view. The offset removes all that drama.
Regards
Arthur.