I can't really answer most of those questions, but I suggest you think of what
slide guitar player has the sound you like most and get a guitar similar to the one he or she plays. Like Bonnie Raitt's (or Kirk's) sound? Get a Strat (or other single coil guitar with a 25.5" scale). Like Johnny Winter's sound? Look for a Gibson (or other guitar with humbuckers and a 24.75" scale).
By the way, I doubt that neck width has much to do with playing slide versus playing without a slide. I could be wrong, but I think that it's all down to what fits your hand regardless of whether you play slide or not.