After marking out (e.g. loco sideframes), to ensure small drills pick up accurately, I normally very lightly centre pop the position, but in 2 stages:
1. with sewing needle in pin chuck, using eye glass accurately position, then press down quite hard to make small indentation (works on plastic, brass, nickel silver and steel provided it's not too hard).
2. with the workpiece supported on a hard flat surface (metal plate?), using the point of a fine centre punch it is possible to 'feel' the indent made by the needle, locate the centre punch and give it a tap with a small hammer. A small drill will pick up on this indent and should run true.
Vertical Milling Slide
I have a Warco WM180 lathe with which I am very happy
warco.co.uk Chiddingford just East of Hindhead, Surrey. No connection, just a very happy customer.
It may be a bit big if you only ever want to make 4mm scale parts, but this is a very rigid machine that also works very well on larger parts (I've made injectors and ejectors for 7¼ gauge locos and even screwcut oversize 3/4” BSPT boiler plugs).
I have a Vertical Milling Slide which is now pretty good, but they need to be mounted rigidly (which they may not be if mounted on the tool post), and as you commented, the 3 screw 'vices' are not very good. I removed the 3 screws and use toolmakers clamps
Rather than mounting on the topslide (the recommended method for my machine) I mounted mine on a piece of 10mm plate and bolted that to the crosslide. I've lost the ability to set the slide at an angle for milling, but gained a lot of rigidity. It's very good for accurate hole positioning/drilling (photo 121) and also pretty good for milling.
Depth of drilling and milling is now controlled by moving the saddle with leadscrew engaged, (instead of moving the topslide). Leadscrew is turned by hand, achieved by putting a homemade handle and card or plastic graduated dial on the end of the leadscrew. I cheat and used a basic single axis Digital Read Out on the saddle movement, which gives excellent accuraccy and saves a lot of counting, but is not essential.
This website might also be of interest for advice on lathe selection http://andysmachines.weebly.com/