5'x3' Ideas and build.

[JAZZCNC]

That's oversize, but there's the idea... When you talk about building in a bit of adjustment to things like brackets etc. - what do you mean exactly? Loose tolerances on clearance holes? That kinda thing? I'm a total noob, man - you'll have to forgive the stupid questions, but any pointers/links to illustrations always helps..!

Little clearance but not too much.! Really I mean adjustable brackets that can be removed and shimmed or with slots etc into alignment making it easier to fine tune.
Building the machine to within 95% of alignment or accuracy is relatively easy the last 5% or less can be a proper bitch and it's this 5% that makes the difference between OK and Great.
In ideal world you'd have surface grinders,milling machine etc and precision measuring and surfaces to work from making building and alignment lot easier.!!
In the real world we tend to have Hack saw, pound shop pillar drill and Big persuader with corn flake box for shims so anything you can do to ease the pain the better.

In an earlier post, you recommended that I go for 16mm/10pitch ball-screws which, at 4 feet, would be running close to their limit - any reason why I shouldn't go for 20mm/10pitch screws, like these?

No if you have the funds then they are perfect size and pitch. Only reason why recommended 16mm was that most folks buy from Chai at linearmotionbearings has he's very very cheap but he doesn't sell them. They are available from other Chinese vendors has Jonathan pointed out earlier but I haven't used them.! (yet)

Incidentally - when I'm getting my plates cut, is it a good idea to get them skimmed to 15mm if they're a little bit over in thickness? You know, to maybe help get any surface 'bumps' out. Or does it really not matter that much?

Yes it helps immensely but to far less degree if the rest of machines important surfaces aren't done to same standard. The important surfaces being, Rail mountings, bearing mounts, ballnut and screwbearing.
Like I said above all these matter greatly for that last 5% and the better the surfaces are prepared the easier it will be.

Hearing you on the extra brace at the back/top of the gantry. Haven't got a clue where I'd fit bracing across the front of the gantry... If I'm bolting through the front of the gantry into the bracing at the rear, then surely that's going to increase rigidity in both directions (albeit in one of those directions it'll be the bolts doing the work...) I imagine that for cutting wood at moderate feed-rates with fairly shallow plunges the stiffness here should suffice without popping bolts...?

Not so much to do with mechanical strength but dealing with or lessoning resonances the machine makes. Any resonance or vibrations will ultimately end up at the cutter giving a poorer finish. If bad it can even affect performance of the machine by throwing the motors into a Cissy fit.!!
Bracing here(See pic blue bit) on gantry and else where on the machines frame help dampen them out.