Would appreciate feedback.

We are building a new house with inslab 1/2 pex. Longest loop is 300' mostly around 9" o/c, the other 5 are between 200 & 300. Am experienced in construction both wearing the tools and in management, estimating and project management.
House is about 1200 sq ft on one level, 50% bermed, and has lots of thermal mass, interior walls are ferro cement etc. roof is r 50 or r 70, last winter we were able to keep the interior above freezing with a wood fireplace when outside temps were -10 C even tho the wall insulation is not complete. On a day that outside temps are 35+ the interior stays around 20 (70 F) with no A/C.
The slab is as follows Well drained sandy soil compacted Double layer poly 2" +/- of ground styrofoam entrained in concrete mix 1cement 2 sand 5 ground styro by volume. 3" foam 1/2 pex stapled or clipped in place.
2" concrete mix 1 cement 2 sand 2 crushed rock. no rebar or mesh, with fiber and super plasticizer for water reduction. because of age (we are in our 70's) we pour each loop in 2 pours 3 days apart which gives us control joints, Pex is sleeved at joints, we keep the latest pour wet for a week or ten days. by then we have prepped for the next pour. Because we are working so slowly we coil the pex needed to run to manifolds and keep it as protected as possible. until we pour the area of floor where it will run (kitchen & hallway etc). Each Pex loop is pressurized the day before we pour and is monitored until the day we need the gage for the next poor. So far no problems, tho reading the thread about the cement buggy made me laugh, we hand carry the cement in buckets so as to protect the pex, or we tip the mud from the mixer onto a piece of 5/8 plywood and rake it, mostly with a wood rake.
I calculate that we will need 35000 btus on a -20 deg day (we are very well insulated) I have gone into a bit too much detail because I have some questions and would like feedback.
I would like to use a modulating tankless water heater as boiler, the one I am looking at has a max output of 144000 btu's but will light at about 1/2 gpm. Will that be a good choice?
Layout is one large living room 25 X 35 a 16 x 14 bedroom an 8 x 9 spare bed/office, 8 x 9 bathroom and 8 x 9 utility room. Am thinking 2 zones one for the lving rm kitchen there is a fireplace on north side with a thermostat against the west wall which is 2X6 wood framed with large windows. The room has a clearstory with 6 3'X3' windows south facing. and the other zone bedrooms and bath, was thinking that two circulating pumps controlled by their thermostats would be the least expensive way of separating the zones, two manifolds on the pressure side and one manifold on the return side. Each loop to have a ball valve which may be used for balancing etc.
Cost is an extreme problem, we seem to run out of money long before we run out of month so are doing
95% of the work ourselves, and have been known to drive 600 miles to save money on insulation. My favorite wife did a rock wall over the fireplace, she does the cement mixing, we do hire a young lad to lift buckets into the mixer for her on pour days.
The feedback I seek is on the boiler, pumps, thermostats, zones.
Very fine site, this is my first post and really hope to get some hints, we have been building for 7 summers now and hope to move in next fall. 2019.