Cheapest tool to make make (or drill) holes into a metal precision straight edge?

Needle files

A Dremel is a brand name for a "rotary tool". I think that Lidl were doing a cordless one for £15 in the last couple of weeks, there might still be some on the shelves. They don't make much noise, neighbours are very unlikely to even notice.

It will spin a tiny (e.g. 5mm dia) grinding wheel at 10's of thousands of RPM. Lots are advertised on eBay if you want to see what one looks like.

I am not a guitarist, but I'd have thought you'd want the tops of the frets to be in a straight line? In which case, there's no need to mess with the straight edge.

You need both. The first thing to do is to string the guitar and use the truss rod to get a slight bow in the fingerboard.. That you need the edge for.

Then you take the strings off and use a vary long flat bar with abrasive to get the frets even and re crown them. You can use straight edge for that, but most people use a sharpie and sand away till all the frets are de-sharpied.

Then you adjust the bridge position and height for the action and intonation

Good heavens! I had no idea. :)