I can only recommend Swiss files (UMV Vallorbe, Grobert, Corradi) above anything. But DICK, PFERD and HASE brands from Germany are quite decent as well. They offer special cuts for aluminium and soft metals as well as files suitable for stainless steel. Vallorbe 'Habilis' files are very nice, they are needle-file styled in the size of smaller machinist's files. Also their 'Valtitan' range is nice. I knew a Us company making (really!) decent files, but I can't find the name right now.
When filing aluminium, use petroleum (lamp oil without smell, or medical oleum petrae) as lubricant/separation aid and the chips will not stick to your file.
When getting needle files: a type that is not included in the standard kits is the knife type. But it is very useful, as it can make a precise first cut when one wants to saw a hard material. Prevents the hacksaw dancing away from the intended line of cutting.
Also get a small triangular scraper for cleaning out corners of openings made by the methods described above. A good scribe is also recommended. Also, the swivelling deburring tools (Noga/Shaviv and compatible ones) are extremely useful. A small one will probably suffice, just get some different blades for it to fit your material.
I personally do not like the automatic center punches. They can jump if not correctly adjusted and make a false impression near the correct one.
Whatever you do, do not allow your files to rub at each other! If lacking the special roll or case, make your own by plastic welding out of heavy-duty plastic bags or roll them in alternate directions in protective paper (brown stuff) or soft cloth.
As stated by other posters, diamond files are limited in their applications. Working well on hard materials. Sometimes stick abrasives (like Degussit) are better, sometimes not.
But the biggest time-saver for making panels of any sort are the sheet metal punches like those offered by Greenlee and others. They can be actuted by a spanner for occasional use and come in a lot of shapes.
For example, the Sub-D connector cutouts and small rectangular cutouts can be made with nearly no corrective work needed.