Easy!
Since your only looking for 14-15 I'll shoot for the low 14 since thats really not that fast by todays standards. With a 240 you should look for a LSD (limited slip diff) from a 300ZX turbo, search the archives for a picture and year info. The ratio will be 3.7 which works well on the street if you want to wind up quicker get the 3.9 rear from an 81-83ZX 5spd. Find a 5 spd gearbox I like the later models ie 82 with a taller first gear then 1-4 are close ratio with a tall overdrive. I like the tall first cause I can pull into traffic, hit it in first and I'm going 40mph in about 2 seconds. Or go with an older 5 spd with wider ratios and a shorter 5th. Either way you will like and need the 5th gear. With the revised gearing you will accelerate alot faster.
For the engine, look for an F54 block 81-83ZX use the stock flat top pistons. Heres where it gets tricky, head choices. Quite honestly the heads are all different and some are much better than others out of the box, but after a good head builder has been over them they all can yield very similiar results. P79 is a good head and very plentiful usually come on the f54 block too. The P90 head is a good choice if you can work it, it will need to be shaved to bring up the compression then the cam towers have to be shimmed and the rocker arm geometry corrected - but this head has the best breathing potential as it was designed for the turbo. I run an E88 that is worked to the point of if it warps I'll have to replace it, since it can't be shaved any more. Both the E88 and E31 are great heads but both need larger valves and port/bowl work. The N47 or 42 is popular but I haven't used them. For more head info check the links that Z racer listed. A stock exhaust manifold mated to a 2.5" pipe with a dynomax, flowmaster, borla or supertrapp muffler is all the exhaust you'd need. If the stock manifold is warped, many are just get a header. You may not see much if any more power but you won't lose any either and the more head work you do the more power you will see. You will need a decent cam. I really can't suggest specs for a cam without knowing what you plan on doing to the head, rpm range you want and the intake setup. Its all a system and works together.
Intake either SU's or a tripple setup since your looking at a 240, the stock fuel injection could support the power and times your asking about though and might be more maintenance free. I tune my webers once a month, usually just need a slight balance adjustment which doesn't take long but is something to consider.
You will need an electronic ignition, the 240 has points which will be a pain on any type of performance setup. You can modify the 240 distributor or just use a newer electronic unit from a 78 Z or even newer 81-83ZX but you will need the brain as well or have to go aftermarket.
Really for the times your wanting the best thing would be to find a donor 81-83 ZX and swap the entire drivetrain with a few tweeks here an there, a cold air intake, open exhaust and gears you will be in the 15's easy.
Matt