I calculated the ratio of the heat loss of the zones to the length of fin tube in each zone. Zone 2 as-is would be able to meet the load at the 99% outside design temp with 140F (or cooler) water according to SlantFin's charts. but zone 3 was still needed more to come into line. If the peak water temp requirement is 140F it means you can run it off a hot-water heater at a standard storage temp.
The Vertex/Polaris would use about 12-15% less fuel than the Combi-2, the smallest forced-draft Burnham cast-iron (ESC3) would use about 8-10% less fuel than the Combi-2.
The size of the pipe makes little difference whether 3/4" or 1/2"- you aren't bumping into any design limits. Adding 15' to zone 5 has negligible impact on the pumping head/flow rates.