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That appears to have the internal flow check, so an external flow check is not necessary on that loop, unless the IFC has failed or was removed or was not included with that circulator pump.

I just installed a Burnhma Alta 136 combi on home that has 11/2 baths. I installed a mixing valve on the domestic side which is required by most boiler manufacturers which make combi boilers. I set the dwh to 140 and mixed down to 120, have had no issues with be able to supply enough hot water.
I personally am not a fan of combi boilers but space was limited.

Is the 160kbtu for 3.3 gpm at your incoming water temp?

The heating part is sized by doing a heat loss calculation on the part of the building to be heated but with a combi it probably is going to be the DHW that will tell you what boiler size you need. DHW is usually set up as a priority over domestic heating so it only needs to meet the DHW need, not both combined. Keep in mind you need the output, not the input to the boiler.

Can you build an attached shed outside the house for a boiler and indirect?

The coldest incoming water temperature is the key to all this. It can be as low as 35F in cold climate, as high as 90 in southern cities.

Well water tends to be around 50- 55 year around.

With those numbers you size the DHW btu required.

With a combi, the dhw load will be bigger, much bigger.

50,000 btu/hr of fin tube output? That is probably at 180F. A heat load calc would determine how often you could run cooler, more efficient SWT.

Our family of 2 gets by with a 120K combi on well water temperatures. A 120K combi turns down to 12K, so the heat load will be easy to accomodate.

Most tankless size be 70- 77 degree temperature rise. So if you want 120F at the fixtures use these numbers to get your gpm. 2- 3 gpm can work for many folks.

140kbtu or even 120kbtu would be fine too, depending on the shower head. A heat loss isn’t needed here, it’ll be oversized no matter what. Use the turndown to help with shortcycling.

Other option would be a boiler that’s a storage tank - so like indirect + boiler all in one.

First thing to find out before doing anything that will cost $$$ is find out if the heat pumps will heat the house during cold weather. You may find you need the boiler anyhow. Glycol can cause issues and with a "few runs of pipe through the outside walls" I wouldn't use glycol due to the expense and trouble it may cause including it turning acidic over time.

I would run the HW system occasionally during cold weather to make sure all the piping and radiators heat. Make sure you have a functioning low water cut off mounted above the lowest boiler safe water line.

Then if the heat pumps prove to be adequate install an outdoor air control to run the circulators when the OA is below 40 degrees.

Your not protected by glycol now so you will be in no more danger from freezing than you presently are now when using the heat pumps.