At 155F design-day temp requirements outdoor reset buys you nothing, efficiency-wise since you can't operate with return-water temps below 140F (a modest 15F delta-T from 155F) without compromising your chimney or the boiler itself. You may in fact have to set it up to run higher temp output to keep it in the safe zone. Outdoor reset only buys you efficiency when it can lower the average operating temp of the boiler (and to some extent the distribution plumbing to the heating system) from say 180F down to 155F. I'm betting that if you've tightened up the place and insulated since the initial system was installed, the baseboards won't even need 155F water, and a different approach should be taken.
The Buderus 115/3 WS has 8.7 gallons of boiler water, making it moderately high-mass. I've never really looked into it's control methods, but Buderus boilers are better insulated than most, and are one of the more sophisticated designs. Still at 85K of output it would be nearly 3x oversized for my 0F heat load, and would run well under it's rated AFUE if run without buffering & heat-purge.
And I GUARANTEE it's more than 3x oversized for individual zones in a 3-zone 1800' "pretty well insulated" home anywhere in MA (or Whitehorse Yukon, for that matter.) At 8.7 gallons you're looking at ~73lbs of water (call it 80lbs to account for the thermal mass of the iron too). With 85KBTU/hr of burner output behind it only takes ~2.5-3 minutes to go from 140F to 180F at ultra light loads, which would be an efficiency-robbing short-cycle. And there's no cheating the physics with a clever control algorithm- you either need more mass, or more radiator in the system. BTUs in must equal BTUs out, or the temp will rise.
What's your zip, and how many gallons of oil/year do you use? Assuming you don't supplement regularly with wood stovers or something it's pretty easy to come up with a quick & dirty (yet still fairly accurate) whole house heat load to do a sanity check on the pro's numbers. Unless they did a lot of measuring it's just plain easier for them to err to the high-side on most of the assumptions, and end up with numbers 2x reality or higher (in fact it's common.) That's why the NORA FSA calculator is so useful- it uses the boiler to measure the heat load.
Even the smallest oil boilers out there have 55-60K of output (more than 1.5x my heat load at 0F), so if microzoning you may in fact be better off with an Ergomax E23 (or the TurboMax equivalent) for the indirect, and plumb it as the hydraulic seperator, slaving the boiler only to the tank's aquastat. That would add 26 gallons of mass to the system, and it would be involved with EVERY boiler burn, not just a hot-water heating burn. If you run the buffer tank at 140-145F water, the heating system and distribution system gets 140-145F water, and the boiler output temp can be 160-170F water and still run at it's max efficiency, since it's the RETURN water temp that determines it's raw combustion effiency. With heat-purge control and cold-start, the standby losses would be less than if you were running it at 155F unbuffered since all cycles would end with the boiler closer to the tank temp, not it's operating full-fire temp, and cold-starting allows the boiler to cool off to room temp if the time between calls for heat are long enough. Since 140F is just fine for a domestic hot water storage temp, it works just fine as a single-temp system, and gives you far more hot-water output than a typical 40 gallon gas-fired unit would (due to the boiler's bigger burner.)
A good discussion of the whys/wherefors of buffering a multi-zoned system to limit short-cycling losses lives here:
http://harscopk.qa.jplhosting.net/uploads/files/harsco%20industrial%20patterson-kelley/publications/4bf77cff8bc8411bb00c67e962d0ef7d.pdf
It's written from a large facility perspective, but the math works the same for multi-zoned residential systems as well. Ideally you'd get the mininum burn time up to 10 minutes or so, but even 5-6 minutes is HUGE improvement over a 2.5 minute min-cycle.
What type of baseboard do you have and how much (zone by zone, if you can measure it)? Combined with a better heat-loss estimate that would tell us roughly what your design day water temp needs to be.
Separate circulators vs. zone valves is a system design detail that isn't amenable to "design by web-forum". There are advantages and disadvantages to both approaches, and which makes the most sense depends on flow requirements & duty cycles, etc.
FWIW: In my own home I micro-zoned it to deal with system balance issues, and ended up running a reverse-indirect as the central buffer/hydraulic seperator. I have it set up for 130F max temp (it kicks on and calls for heat at ~122- 123F), and used a low-temp tolerant gas-fired on-demand hot water heater as a modulating low-mass boiler. With all zones calling for heat with someone taking an endless shower the burner never hits more than 50-55K of output, and everybody is happy. I also have a drainwater heat recovery heat exchanger delivering 20-25K to the incoming cold water stream for the hot water coil, but we're still only talking 80K max input, ever. During heating-only calls it's never putting out more than ~42K (when serving the fan-coil on an air-handler zone). With the modulating burner the min-cycle is about 11-12 minutes only relying on the 7-8F of hysteresis on the indirect's aquastat, but that hysteresis could be increased if need be, with some retrofit controls. But with the modulating low mass burner it wasn't necessary.