If you put a buffer tank on the boiler loop of the P/S system it'll contribute some standby loss when operating in water-heating-only mode. It's best to put it in series with either the supply or return manifold of the heating loops only. But in this climate the heating season is 8-9 months long, so the small efficiency hit on water-heating-only would only relevant in the summer. If it would shorten the plumbing to both the indirect & buffer by having the buffer in the boiler loop it's at worst a wash- the standby losses from the less-insulated plumbing can easily exceed standby losses through the walls of an insulated electric HW heater used as a buffer.
The "ideal" spot for the buffer would be to have the closely spaced Tees for the zone supplies right at the boiler-output connection to the tank, and the zone return tees connected close to the boiler return, essentially making the tank the point of hydraulic seperation, something close to this:
(You may find this a useful read: http://www.radiantandhydronics.com/A...00000000597932 and this http://www.radiantandhydronics.com/Articles/Glitch_and_Fix/BNP_GUID_9-5-2006_A_10000000000000201092)
But if it's just in series with the boiler output (or return) and the manifold to the zones the it'll still stop the short-cycling.
With fin-tube 110F would usually be too low an output for consistent results at low heat loads, but see how it does. It might be OK until dust-kittens build up in the convectors. Boiler output of 180F @ 0F is probably more than necessary for the actual (as opposed to calculated) loads with 125' of fin tube, but about right if your load is truly 70-75K as opposed to the built-25% margin of many calculations. If the 0F heat load it's actually closer to 55K (likely) you'd be able to keep up with a curve that crosses 160F or 165F @ 0F outdoor temp, and have more condensing time. Even when operating above condensing temps, you reap ~2-3% fuel savings for every 10F you can peel off the water temp. With a condensing burner those savings are something like 8% when dropping from 140F down to 130F though, and you get another hefty chunk dropping the output down to 120F, so setting the curve as low as it can be and still keep up at design conditions counts. Using the boiler to measure the heat load is far more accurate than any Manual-J or IBR type calculation.