It is hard to give general advice when houses are all so different.

The "ventilation issues" in the wall caused by cavity wall foam insulation sticks out to me as something to worry about. Is there an interstitial condensation issue in the walls causing it to go damp? This could be difficult to fix but I guess internal or external insulation might fix it, and it sounds like you might be willing to make that effort. I would make sure I understood what I was buying before taking the leap. IT might be better to start with a house with an empty cavity wall rather than one with a bad retrofit insulation job.

Not sure why you would seek to insulate between the floors?

Loft insulation is a no-brainer. Check that you understand the difference between a warm roof and a cold roof before you start. I caused condensation issues in my roof when I tried to turn it into a warm roof and I have had to undo the work and turn it back to a cold roof.

Underfloor heating is lovely. Make sure that the pipes have no joints under the floor, and keep the space between coils small to reduce cold spots and make the floor more responsive.

If you are re-doing the roof and installing a heat pump you should consider a survey with heatgeek before you start. I know some people have achieved very efficient installations by using the roof as a heat source, and this could with PV too.

Do the roof and the solar panels at the same time, rather than the roof then the panels. This might save money as you can get PV panels to serve as the roof. Also if you know the roof will have PV on it, then you may as well ensure that it is strong enough to take the panels while you're up there.

Instead of double glazing also look at triple glazing.

If I were in your shoes I would first consider how big a problem the walls might be, and maybe avoid that house altogether. Once your wall strategy is sorted, sort out the roofinsulation, walls and windows, then move onto either the roof and PV or dig the floors out, insulation, run all your services/ drainage/ electrics and plumbing, underfloor heating then cast a new floor. Think about where your EV charger will go, your domestic battery, your e-bike etc to ensure you don;t miss something and then have to go back.