Let the fun begin !!! ???

[JAZZCNC]

At this length then go with 16mm you'll be ok, 16mm will allow slightly faster acceleration due to less inertia. Resolution with 20mm isn't a problem because the ratio effectively makes it same as 10mm and even with 1:1 ratio 20mm res you'd still have enough to cut the mayan calender.

There's very little between the 3Nm and 4Nm motors in terms of performance so you wouldn't really see much differance. Either will work fine with what your planning to use. Just run them around 70Vdc and you'll be fine.

With bearing blocks/ballnut then all you need is to leave a enough clearance in case you over run limits. In which case the limits should stop the machine so you need enough clearence for the over run. That said your better fitting hard stops in case of over run and have the bearing blocks just past these.
To be honest I wouldn't sweat over it you can control this with adjusting limits and they take a fair old bang to damage anyway so worst you'll do is knock something out of alianment.

Let me just say if your at the point of planning in cad to this level of detail then your wasting time that could be better put to use building the machine because in reality you'll more than likely to see a better place to put them on the actual machine.!! . . . . . Time to put the pencils down and pick up the tools.