I suppose it depends on what else you want to change. Stan is correct in that the "best" base block+head combo that is easy to source is the M30B35, which was in cars starting with the 88 model year. This combo will give you the best out of the box performance, with hp and torque above 200 even with the stock Zeniths.
However, if you really want to wake up the powertrain, adding the Motronic FI to the mix at the same time will make the car vastly easier to live with. It can also give you a bump in power/torque and fuel economy. There are 2 versions, 1.0 (as in the 533) and 1.3. 1.3 is the superior version and it lets you use the CS flywheel, which is much lighter than later dual-mass units.
If you want to go crazy, there are 2 more options; the M90+B35 head combo or the S38. The M90 block, with rods+pistons+B35 head can give you ~3.7L, a 9.3:1 CR, (or whatever you spec if you go with custom pistons) and an oversquare engine, which makes it happy to rev. However, M90 block are becoming rather rare, so this is not the easiest path.
In terms of the 533, the E9 oil pan will be needed, as well as the pump. (as I recall, the pickup depths were different) Motor mounts are compatible. The 5 speed will be a Getrag 260 (not bad, but not the one you want, which is the 265) which does not have a compatible speedo cable drive gear. (not too sure about that one, moved mine from the stock 4 speed to the 265)
