For the 5 lug swap, here's what you need...Rear bearings and hubs from a 300zx or s14se, front spindles, washers, locknuts, strut bolts/nuts, and hub from an s14se. You also need to remove the s13 balljoint from the s13 lower control arm and put an s14 balljoint in its place. Several aftermarket companies make 5 bolt hubs for the s13 which aren't too steep in prices. Check
http://www.splparts.com and
http://www.pdm-racing.com. Never heard of a q45 vlsd going into a 240sx. But I have heard of r230's from a 300zx being able to fit with the 300zx rear cover and 240sx vlsd differential outputs. I hear that due to the taller gearing in the r230, this differential makes for a sluggish 240sx. Try it out and tell us how it goes. I'm not sure of the q45 suspension but it probably won't upgrade your s13 even if it swaps. The s13 shares some of the rear multilink bars with the 300zx, so if this is also the same in a q45, it should work. The q45 calipers and rotors bolt on to the s13 no problem though. hope this helps.