The depth of the box will work OK. When it comes to fretting it, just use as many as will fit on the fretboard!
If you are using a floating bridge, it will be simply held in place by the pressure of the strings.
Have you seen anyone just wood burn frets in? Would that work?
perfect for slide....it's been done many, many times. You still need to be careful to get a level fretboard if you plant on playing any fingerstyle. possible on fretless, but more difficult.
For a first time build I'd recommend fretless with simple fret markers - putting frets in is not the easiest thing to do. It's not rocket science, but there is a craft and a knack to it..so I'd suggest you go fretless for a first time build.
I always plan for at least 17 frets, and however many more look/feel right. 17 allows all the first position picking to be played 1 octave up.
woodburning in fretmarkers looks great.
A floating bridge is a good idea, it allows you to shift & angle it to compensate for string stretch and get the intonation correct (look at a commercially produced guitar, the bridge sits at an angle with the bass string side farther away from the nut).
but not a ROLLING bridge. a bit of threaded rod just sitting there will tend to roll forward out-of-position toward the nut as you tune/play/retune.
All three have their merits. With oil (linseed, Danish, TruOil, etc) you can get a very nice & durable finish. A bonus is that in time, if the neck gets worn, a simple re-application of oil is all that's needed.
With lacquer, ambient temperature and proper ventilation are important for health reasons. Plus the drying/curing time can be from a few days to a few weeks depending on relative humidity, temp., etc.
Here is a link for fret calculation...http://www.stewmac.com/FretCalculator/
With the box you mentioned being 9" long, wanting a 25" scale length, if you position the bridge about 2" from the tail end of the box, there will be space on the neck from box to nut for 18 frets, either wood burned or actual frets.
Best of luck on your project.