Aluminium Profile Type Router - Initial questions

[JAZZCNC]

If these are joined simply by using the side gantry plates and socket head bolts to hold the pieces in contact is that a terrible idea? Could it be enhanced with some strong adhesive between the pieces to compensate. Really wan't to avoid wasting money on the extrusion if I can help it.. Any ideas?

It's not the best of ways but if you bolt the profiles together along there length as well as the ends it should be ok. Obviously, because the slots don't line up you won't be able to use the slots to lock together but you should be able to drill and tap them together from underneath with a few bolts.
Don't use adhesive.!!

For the blocks which will mount to carriages - Is Ecocast type aluminium recommended here? Quite pricey for 50mm thick block so want to be sure it's worth using this.

Cast plate isn't required but the plates must perfectly flat with parallel faces and exactly same thickness on both sides of machine. Rolled plate isn't flat or with parallel faces so unless you machine it flat and parallel then ground plate is your only choice if you want it right. Why 50mm thick.?

Also thinking of what you mentioned Jazz - adjustability. Would slightly oversize holes be the usual way to achieve this?

Try to keep the holes to correct size if possible. Only make oversize if needed.
When I say adjustability I mean don't machine plates with slots etc that lock things together unless the whole machine is built this way and on an accurate machine. Adding plates or brackets that allow adjustability is always a good idea. When making plates etc then adding slots rather than holes can help in some places but depends on machine design.

One of my methods is to use slots to allow adjustabilty then when it's all setup I'll lock it in place using tappered holes and pins.

On electronics - I'm considering Hybrid steppers now, certainly on X due to the dual X motors and potential stalling. Any recommendations on models here?

Wouldn't use anything else. These work well and allow AC or DC with voltages upto 70Vac or 100Vdc. This means you'll get good speed and with 5.5Nm you've plenty of power.
If you go for the lower powered motors just check the drives Max voltage because a lot of them are 50Vdc Max which will limit your speed.

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