What's the best way to go?
Best is probably gravity flow to adjacent elevated storage, but first you need top of storage high enough above top of the boiler, fat enough pipes, and a low-resistance flapper-type check valve, ball-type check valve, or automag type valve.
If automag plus finned coils is easier or more cost effective then that is the best way to go. Either way gravity based solutions are what is required, as Fukishima has shown.
The hard part for me was the check valve. Loading units come with just the right check valve for free, but my budget didn't include a loading valve assembly. I built one by replacing the bronze flapper of a conventional swing check with an equivalent part machined/carved out of HDPE. Works great but it took half a day to build, test, and deploy.
As for high enough storage and fat enough pipes, the calculations are pretty straightforward. For mine I have 30 inches from top of boiler to top of storage plus about 36 inches of boiler with a vertical tank. I assumed the boiler was equivalent to 18 inches of return temperature water plus 18 inches of supply temperature water.
For worst case design I figured that storage would be fully charged to 180 degF (and therefore 180 degF return temperature) and the fire going full blast. Assuming a supply temperature of 220 degF I went and figured the weights of all the one-square-inch columns of water involved and came up with a three tenths of a psi of head or whatever and used that to figure the flow through the boiler, pipe, and elbows. I came up with a little over 4 gpm, which would be 80000 btu per hour on a good testing day, so I figured it was worth a try.
Got the system up and running and got storage up to 180 degF top to bottom with a good big fire and a nice deep layer of coals and cut the power. The supply temperature peaked out closer to 230 degF but it seemed to ride it out just fine. Once the draft fan was off things calmed down pretty well.
