I may have over-stated the cost-effectiveness-R values somewhat. Another opinion has R7.5/R15/R30/R65 for slab/foundation/wall/roof- see the minimums for zone-5, Table 2 on page 10:
But still, the rigid iso was the expensive part, and you're already committed to adding the studwall- the ecoomics of adding batts still make sense.
Wtih the buffer tank piped as the point of hydraulic separation between primary/secondary loops the boiler flow rate is independent of the heating zones' flow rate(s). The boiler throttles back more and more as the return water temp rises, and at some point the tank temp is high enough the boiler's controls kill the flame, even as the flow to the radiation continues, even tually lower the average temp of the tank. When the boiler re-fires, much of the output water from the boiler short-circuits across to the heating zone loops, reducing the high rate of mixing that would otherwise occur in the buffer. But that's just one common method of doing it. Yours is a simple/small enough system that it might be fine to simply put the tank in series between the boiler output and the zone-manifold. The pumping & piping details can and WILL differ.
A real design involves doing the math on the flow requirements & pumping rates for the zones, the pumping rates required by the selected boiler and the relative head- pressures, etc. Unless you want to take the course in hydronic design or home-study it, you really need to find the right system designer/contractor. This is not and will never be a design-by-web-forum type of problem except in the simplest of cases (and even then, you often only get what you pay for.)
Any contractor who still thinks ANY fin-tube requires 180F water is stuck in the 1950s, or didn't take the course, and surely never read a fin-tube spec sheet.
Many old school systems were DESIGNED to require 180F water to be able to be able to deliver the load AT DESIGN CONDITIONS (with a lot of padding built into the calculations- most could still work even then with 150-160F water on design-day), but 99% of the time it isn't -2F or 0F outside, and the heat load is a lot less. At ~35F the heat load is half the design load, and even in a minimalist system that required 180F water at 0F outdoors could keep the place cozy with 140F water.
A: The thing was probably overdesigned even for the leaky barely insulated 1950 version of the building with single-pane windows
B: You have fuel-use evidence that your true design-day heat load is closer to 30-35K and may be even less, a load easily met with 135F water in 145' of fin tube
C: You've cut the heat loss by probably 30-50% with your already-done insulation and air-sealing upgrades, and you're adding more insulation to the foundation.
Assume it was 50% overdesigned on day 1 in 1950 with 180F water (it was probably even more than that- 100, 200, even 300% oversizing isn't uncommon), and the design output for the 1950 house was in fact...
90K which is about the right output figure for 145' of fin-tube @ 180F (and suspiciously close to the 'heat loss" submitted by one of the contractors.)
...which means the real heat loss back in 1950 was really only....
If you've cut down that 60K d heat loss 30% with insulation & better windows (it's probably more) it means you'd be currently looking at...
... as your actual design condition heat load.
That's less than half the 180F output that it was originally designed for, and a BTU-rate the 145' of fin tube can deliver with sub-140F water. And that's on the COLDEST hours of the COLDEST days. Most of the time it can run even cooler boiler output. This is a VERY common scenario.
If it was even more oversized in the beginning (likely), or your improvements to date are better than 30% (also likely), you'd never need more than 125F-130F water to meet the heat load even during the coldest hours of the year.
Fin tube "sheds heat" primarily by convection, and it's output is very roughly (not perfectly) linear with the difference in temperature at the floor (~65F) and the water temp. At 180F that's an air-water delta-T of 115F. With 135F water that delta is reduced to ~70F, which is ~60% of the 115F delta, whtile the output is more like ~50% of the 180F numbers. (see the above linked short-spec for a particular fin-tube.) Because the fin-tube is relying on induced "stack effect" air flow for convection, that force gets to be really weak given the short height of baseboard, which is why it's tough to get reliable results with sub 120F output water in most fin-tube.
But even 130-135F water out will return 110F or cooler water if you set up the flows correctly, delivering low-mid 90s efficiency with a mod-con. From the description would appear you have sufficient baseboard on your existing zones to be able to run it there MOST of the time, maybe even ALL of the time, and a contractor with design skills can make that happen. (But surely not the guy who believes "Thou shalt deliver 180F water to baseboard" was engraved in stone by the hand of god.)