1) The best fit engine for a 626 would be a series 4 or 5 13B, since the engine mounts are in the middle of the engine opposed to the series 1 to 3 engines which mount on the front cover reducing the fabrication work. The ecu can be swapped out with a carb and distributer from the older engines.
2)A 1st gen diff can cost from $2500 to $5000 depending on condition and if you can find one. The 626 rearend and 1st gen are the same so if you can find an LSD guts and swap it out that would work.
3)1st gen dashboard?....... keep the old dashboard and spend the money you would on a 1st gen dashboard, on some gauges. Some fibreglass and upholstery could help fix the old one if needed.
4)GSL-SE suspension would be best swap for your car but will be costly.
Redrilling can work or you could use a conversion kit from RE-SPEED
http://mrcmfg.com/catalog/index.php?cPath=112_22_73
But you will need the 2nd gen brakes for this.
MAZTUNE should still have a few.