Fret press arbor and caul -- not essential
Fret bender -- need something to do this job... the one I used was a DIY version (I have a "real" one now, but the old one was adequate)
Fret end dressing stick -- not essential
Fret leveling file -- not essential, but need something that will do this job
6" Fretboard leveler -- not essential
6" Fingerboard leveler -- not essential, but if I were to buy just one non-essential tool on this list, this would probably be the one
Double edge fret file -- need something to do this job... some sort of file, and I really like this one
Fret end dressing file -- need something to do this job, some sort of file... and this one is nice because there's a rounded edge that won't mar the fretboard
.023" kerf Dozuki saw (for fret slots) -- unless you're going to get a preslotted board, you need a fret saw. I used Stew-Mac's saw first, but it didn't cut well for me. This one is 10 times better, IMO.
Spokeshave -- not strictly speaking essential, but I would not build a neck without one. You could do it with just rasps though.
Router fretboard radius jig -- not essential, you can do the whole job with a (preferably 16" long) radius sanding block
Fret saw miter box & 25" template -- not essential, but it makes life a lot easier. You could cobble something together on your own that would serve as a miter box.
16" long, 12"-radius sanding block -- I'd say essential. Substitute your radius of choice.
And a good flat-edge.