Good, so loosely you are in area where the ground is flattish, and with no easy to reach substrate.

I would say that basically you can go for a "light" (in weight) building (think of mainly wood) that will be "floating" or a traditional (or innovative) masonry one (that will likely need some deep foundations or at least an underground floor).

If you go for the first, there are firms that make pre-fabricated houses that are on par (or better) than anything you can build locally, the most re-known one (but of course not the only one) is HufHaus:

https://www.huf-haus.com/

they do have some operations in Belgium:

https://www.huf-haus.com/en-uk/europa/belgium/

AFAIK pre-fabricated houses of this kind - generally speaking - are not "cheap" but very good quality, though ultimately you get what you pay, so there are whole ranges of them, this is near the top (both for quality and price).

Having an underground floor can be an advantage (lots of space for technical apparata for ventilation, heating and what not, storage space, etc. and a good insulation from humidity.

Then the structure.

Costs (for a 1/2 storey) shouldn't be much different between an (again pre-fabricated) steel frame and a (cast on site) concrete frame (pillars and main beams).

Structural masonry should be slightly more expensive.

Decks can as well be concrete or steel (but you can also have wooden beams if you like them), cost should not change much between the types, unless you are going to have large "open space" rooms, in which case usually metal decks are cheaper.

What changes are the building times, metal structure is the fastest, concrete is second, traditional (or innovative) masonry third (think roughly of 1-2 months vs. 5-6 months vs. 8-10 months).

If you have a steel or concrete structure you are free to "wrap it" with practically any possible exterior wall/finish, with masonry you are a little more limited (in practice either esposed brickwork or plaster (there are also special wallboards suitable for exterior, but they are rather expensive).

But we briefly covered only the very basics.

What you have to understand is that a building is a complex assembly that - for some aspects - is additionally like a living being (it does "breath", it expands and contracts, it absorbs and releases humidity) so you cannot pick "arbitrarily" this (or that) construction material or method without considering how it integrates with the rest of the building.

What you need to choose first is a (good) architect and/or engineer and talk with him/her.

Such a professional should be - besides a technologist - also a sort of counselor that should guide/assist you in taking the "right" choices.

What I can tell you:

1) nowadays it makes sense when building a new house to have it higly efficient (thermically/energy wise) but you have to know that you are going to pay in advance in construction and materials the energy costs of what you will save in the next 15-20 years (i.e., example, if you will save 2K Euro/year on energy, this will have an added construction cost of at least 30K-40K Euro)

2) if you are going for an energy efficient house (class A, A+ or B) you NEED a PROPER mechanical ventilation system (easier to say than to design it) and you will have to consider its maintenance costs (filters and heat exchangers need to be cleaned periodically, fans motors may need to be replaced, controllers, like all electronics, sometimes do fry)

3) insulating materials - besides their costs - carry with them a number of "consequences", hay and rammed earth needs an exaggerated wall thickness, wool, wood and paper are hygroscopic (as well as rock wool) polistyrene (in foam boards or sprayed) is totally impermeable, personally (but of course it depends) I "believe" in other materials, like PET mats like:

http://www.maiano.it/eng/products/construction/sintherm.html

or cellular glass, like:

https://www.foamglas.com/nl-be/producten/

4) everytime you "deviate" from what is "commonly in use" in your area it will cost you something more or it will take some more time (either because the material will be more expensive or because its assembling will take more time because builders are not "used" to it)