I've built a couple lap steels myself - what features you want will depend a bit on what style you want to play with that lap steel...
For blues and rock stuff, I'd use 6 strings, tuned to open G, open E, or a similar tuning, and I'd use a P90 or a humbucker, or maybe, if you don't mind the price, something like a Lollar Chicago Steel; I'd also think about using two pickups (instead of the traditional single PU setup) - a neck PU on a lap steel is great for bluesier solo playing...
For "classic" lap steel sounds like Western Swing and Hawaiian music, but also for country rock, Americana, alt.country style stuff, I'd go with 8 strings tuned to C6 (or maybe A6), and use a pair of Stringmaster-style singlecoils (Lollar or VVG make those), or maybe a CharlieChristian type PU.
As for scale length, personally I like something in the 23-24" range - the traditional 22 or 22.5" (in conjunction with a string spacing that is equally wide at the bridge and nut) is great if you use a lot of bar slants in your playing; if you don't use slants at all, a longer scale (25-26") will give you more sustain - 23-24" is a nice compromise that still allows slants, and will sustain quite well.
If you are love pedal-steely sounds, you might think about adding a Duesneberg Multibender to you lap steel; this works great with a 6-string E6 or E7 tuning, and the bender levers on the 2nd and 3rd string set up like the A and B pedals of the pedal steel.
I would definitely include volume and tone pots, and I'd place them so you can reach them with your pinky - for doing volume swells and boo-wah effects; since you rarely strum on a lap steel, placing them close to the strings is not a problem...
