Hydronic, In-floor, Radiant Heat
Lots to answer here.  I would have posted this as a comment, but I have too much to say.
I have an 8 zone hydronic (in floor concrete) heating system...probably &quot;over-zoned&quot;, but that's a different issue.
When you say &quot;the manifold&quot; that implies only one zone. I have 8 manifolds.
PEX is awesome and will last a lot longer than 35 years.
There are a lot of considerations when designing an infloor system. IE: what is your heat source? Electric boiler, Propane boiler, Natural gas boiler?  Air to water heat pump? Geothermal heat pump?   I love my in-floor heating, esp, during the winter, but bear in mind it's SLOW SLOW SLOW to heat up a space. You can't do typical night-time setbacks on your T-stat by more than a couple of degrees. Also, of course, in-floor heating doesn't give you A/C.   My house is set up with an air handler and ducting for A/C as well as aux heat for when the in - floor heating system can't get up to set point.  That's done via a fan coil from the boiler to the air handler and is controlled by the T-stat as Aux-Heat.
As I've said here before, and hate to say it bc this is a DIY site, but you should at least consult a pro in this field to give you a design, with tubing layout, temp loss calculations...you don't want to screw this up...it's hard to fix later.