Cool project!
3D printing will do very nicely, both in strength and accuracy. The FDM printed sample you have is not the neatest ever but does allow you to check if everything fits and 'works.'
As you're messing around with gear meshes - plus a part that (even if it doesn't hold the shock tower or turnbuckle joints) is exposed to quite a few stresses, I would go for a stronger material - and an accurate one at that. CNC Machining is indeed quite expensive - and in fact, may be unnecessary:
Option 1: Have the parts printed in a durable printing material (Nylon printed in an 'SLS' printer for example) - I predict that would set you back 30-40 euros and is just about as strong as nylon of the same blend when moulded
It is also very accurate. The only potential problem is that the material is slightly porous, so it might not repell all the dust and dirt. This though it easily solved by coating the thing on the outside with paint or glue.
Option 2: Have the casings printed in an extremely detailed and smooth material - it will look moulded and smooth as silk! That's a good thing, because then you can make a silicon mould from these parts and cast (or vacuum mould) parts in these silicon moulds and resell them
It will be more expensive for a single set, but if you do 10 or 20 sets it's already money well spent. A downside to this is that any mods/customizations along the way will have to pay themselves back.
I work at a small company nowadays that does research on polymers and 3D printing, so if you are looking for more information, help finding the right material(s) or even refer you to contacts in the printing business to get the parts made, don't hesitate to PM me
Edit: Am I seeing correctly that the front gearbox mounts are the limiting factor to lowering the motor any further? If so, perhaps it's an option to entirely skip these mountings, incorporate TLR4162 (the inboard motor guard) into the gearbox housing and use the chassis holes for that part as the front mounts? Or the holes beside the front suspension hanger of the rear suspension that are unused in the mid-motor setting?
It requires some re-thinking on the mountings of course, but if successful any concerns of adverse handling effects should be off the table