The double wall ones are almost certainly Chinese and the most popular single wall ones are made by Kingspan, Viessmann, Sunda and a couple of others. Also, there is a relatively new one on the market from Narva in germany which should last a long time and be quite efficient.
It will be no problem to have them all in series but the flow rate should be higher, per m2 of panel, so that the terminal temp is kept reasonable.
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