400mm Z stroke is way too much for a typical gantry mill. This is an area where 3D printers and CNC routers differ. The spindle takes up a lot of vertical space. Your gantry would have to be gigantic.
How are you going to make your own? Your drill is a Black & Decker? This is not a good sign! There are frame kits for 3040 and 6040 type machines on Ebay. Making your own BOM for everything that would need to be added to such a kit: that is doable.
There are engineering compromises that make selecting the right components for a hybrid machine difficult. It's going to be either: 1) very expensive, 2) very slow as a CNC, or 3) very sloppy as a printer.
Also, I really think that with 3D printers... people talk about frames a lot, but the important things for actually getting the damn things working are the extruder mechanism, the heated bed, and calibration (auto or otherwise).