Size the indirect for the biggest tub you have to fill. The 40 is probably going to work unless you have a big soaking tub. Recovery performance on indirects is mostly about the burner size of the boiler it's hooked up to, which in your case is more than 2x that of any residential 50 gallon standalone. Hot water performance of indirects is SO good you can usually downsize to 40 gallons from 50 gallon standalone, unless you're hooking it up to a very sub-50K boiler.
A new boiler + indirect will likely be $8-10K at the very low end for cast iron, no frills or system tweaks, to $12-20K for a right-sized modulating-condensing boiler with the system carefully re-configured to take full advantage of it. A conversion burner, $2-4K in my neighborhood.
Adding outdoor reset boiler controls only makes sense if the system radiation is sized to actually need 200F+ water on design day. When you've done a bunch of upgrades to bring the load down the water temp requirements for delivering the heat is often even below the safe RETURN water temp for a non-condensing boiler (130F if burning gas, 140F if oil), which means the reset control is always stuck at the low temp end, and the thing just cycles within a narrow temperature range. In those situations you get a lot more out of an Intellicon 3250HW+ or similar heat purging economizer ($200 for the hardware as DIY, $500-700 sourced & installed by a pro.) Rather than keeping the output temperature at particular range, they work by analysing burn times from recent calls for heat to "learn" the system, turning the burner (but not the pump) off prior to the end of a call for heat, dumping the residual heat in the boiler into the zone. If the boiler is above it's programmed low-limit on a new call for heat, it inhibits the burner from firing until the low-limit is reached. By always parking the boiler at a lower temp, the standby losses are reduced, but by allowing the boiler to rise (to 180-220F if need be) during the call for heat, then purging that heat into the zone at the end of the call, the number of cycles and cycling losses go way down too.
Re-jetting the oil boiler to 100K will likely save ~10-15% if your heat load is truly 82K (I'm skeptical that it's that high, but OK.) If you do that you'll likely have to tweak the plumbing to keep the return water above 140F, and you'll need a stainless flue liner. If it was installed 10 years ago it may already have the liner, and it may be a model tolerant of lower return water temps. If you have the manual , see what it says. If not, what's the model number?
Either way, if oil stays above $3.50/gallon the conversion burner option pays off in short years, and you can probably get another 15 years or more out of the sucker if it's still in good shape, and revisit the replacement options then- you may have dropped the heat load to under 50KBTU/hr by then. Installing an indirect at the same time as the conversion burner would be another $800-1200 in my neighborhood. YMMV.
At $3.50 oil heating with the mini-splits costs less than half- comparable to heating with gas in an 80% boiler in most markets. If the older part of the house doesn't have air conditioning and the floor layout fairly open, adding another mini-split and only firing up the oil burner when the heat pump doesn't keep up might be the right thing to do. A 2 ton Fujitsu or Mitsubishi can typically pump out on the order of 30,000BTU/hr @ +15F outdoor temps (the 99% design temp for your area), and may cost less than the total cost of conversion burner + indirect + economizer control. If your peak load for those zones is 50K, a 2-ton ductless would more than cover the average winter load, reducing oil consumption by more than 80%.
A standalone gas water heater or a better class heat pump water heater like the AirTap or Stiebel-Eltron is dramatically cheaper to run than an indirect on an oil-boiler at current fuel & electricity prices.