Lowcost capable CNC - Make vs Buy?

[JAZZCNC]

Mechanics
Ballscrews all round, but only 3
Single Y ballscrew will save a bit of budget, however I do need to cut ends of boards set vertically. With an under-table ballscrew and cross brace, I will needs to have the spindle sitting forward of the bearings enough to do this. I only need 65mm of cut distance beyond table. Is that a sensible design option?

Single or double ball screws it doesn't make any difference to if you can overhang or not. This is down to gantry design.
Many will sweep the gantry beam back from the centre of the bearing plates to recover lost tablespace. However, in your case this can't be done because you need the spindle to project past the end of the bed so your better with the gantry beam inline with the bearing plates.

There are other design methods, like having the long axis frame rails extend past the end of the bed. In this way you can have both methods and have any overhang amount you like(within reason). The cost is it takes up more physical room and slightly longer rails which cost more money.
I've used this design on several machines so that a 4th axis can be used on the end for turning and fluting etc. The same can be done with extending the gantry at one side and using the length of machine for longer work. (See pics)

Gantry

I get the material in the slots to lock the two extrusions together, but see that 10mm aluminium bar is not a standard size so what to use? Options seem to be either machine a ½” aluminum bar down to 10mm slot size on a router table or forget the slot filler and have multiple bolts. By creating a gantry length ‘washer’ with regular clearance holes along it, I could use it as a guide to drill the access holes in the lower extrusion, then install bolts in it to mate with slot nuts in the upper extrusion, so they are bolted every 50mm or so. Avoids having to machine material to tolerance?

Your way way over thinking this.!! . . . For machine this size it's not required. To be honest at this size you would get away with NO bolts. If not using the spacer method then 2 x M10 bolts spaced evenly along length and tapped into the upper profile will be more than enough.

Electrics
Tempted by closed loop steppers - looks like for 2nm nema23 steppers it is about £40 stepper plus £20 driver vs £80 for closed loop motor and driver, so maybe extra £20 per axis well spent. Where will I see the benefit besides getting error rather than inaccuracy due to missed steps?

Depending on which you get then the main differences will be smoothness and speed/torque.

The smaller closed-loop motors (<=3nm) all tend to be 3 phase and the motors use 0.9Deg step amount or 1.2 rather than 1.8deg found on typical hybrid steppers. This gives a smoother action but requires more pulses from the controller to get the same speed. But controllers like AXBB or UC300 etc have more than enough frequency so this isn't an issue.

Being 3phase they provide more torque and allow higher rpm's. Typical RPM where torque starts to drops away is 1000-1100rpm for hybrid stepper motor. With closed loop it's around 1500Rpm, but this does depend on other factors like quality and voltage etc. But in genral they are faster and stronger.

Larger (=> 4Nm) Closed-loop motors can be found either in 3phase or two phase versions.

But still all down to the design, so guess I need to fire up Fusion360 and start designing. Plan to start from the Z-axis and work out axis by axis. The Z seems a simple matter of sandwiching 15mm linear rails and 1605 ball screw between two plates and getting the heights and clearances right...we shall see.

Don't use 15mm for the Z-axis as it makes more difficult to build regards clearances and they are fiddly. 20mm cost very little extra and make for a much stronger and easier to build Z-axis.
Whatever you do don't cut corners on the Z-axis, it's THE one area you don't want to get wrong because it holds the tools so if it flexes or vibrates this will be seen in your work.!


Below are 2 machine designs, both can be single or twin screws and placed under the bed for protection. The larger one will extend past the end of the bed and the side, it also adjustable length 4th axis.

Edit: also a larger version with extended side rails for more front extension.