Hi Paul, That shouldn't be too difficult to build but does depend on exactly how tight the budget is.?

Plasma and Drilling/tapping by there very nature don't lend them selfs to budget machines. If you want reliability and hassle-free plasma cutting then you'll need to buy a decent Plasma unit like a Hypertherm. The cheaper chinese units will easily do the job of cutting 2mm box but they are not all just plug n play and will require more effort to get working reliably.

From the sounds of it, you'd be better with a rotary unit that you can cut/mill all 4 sides. The Plasma side is easy enough as it's just mostly the plasma unit and possible torch height control but the Drill/tapping will require the correct spindle type with high torque at low RPM with either a tapping head or an encoder on the spindle to allow it to do ridged tapping. The controller will also need to have the ability to read the encoder and do ridged tapping. This will push the budget up as slow-spinning motors with encoders are not so common or cheap and neither is a tapping head.
There are several options on controllers ranging from Free to very expensive and all with different levels of complexity and learning curves.

Bespoke hybrid machines like what your planning are not really something that fit with "Very tight budgets".!!