sierra's have never been released with a 1.6L, only the vitara's (track-kick). all sierra's had either 1.0L, early models, or a 1.3 either carb, or injected. Even the Jimny's come with injected 1.3's.
a lot of people put the G16a (carby) or G16b (injected) motors in their sierras, as its not a hard swap. there are bell housing kits available to buy here in aus for the 1.6 conversion, using the sierra 5 speed.
as for which one to modify, a lot of aussies will say the sierra is better to mod. leaf sprung, simple, easy and cheap to mod. a simple SPOA will easily run 31"s, a bit of guard trimming or bashing can see up to 33"s running.
it all depends on how you drive, what you want to drive, and how hard you're going to do it. like rock, sand (beaches), mud etc. are you looking at an all-rounder? or a hard core basher? mega flex?
6" lift? why would you want so much? a SPOA is anything from 3"-5" depending on how you do it. (either as a kit or a DIY style) will this be including BL or just straigh suspension lift?
tri y headers (extractors, 4-2-1) are a good mod with a 2" exhaust with a good cat and muffler. will improve top end and response a bit.
Gears? diff gears, or transfer gears? diff gears will let you run 31"s as if it were stock. transfer gears for reduction mania.
lockers? well, if ur getting transfer gears, you might as well go twin locked front and rear. your zook can only go as far as the traction it has. are you planning to weld the centres? (cheap) lock-rite locker (good) or air lockers (best)
carby is a simple engine, easy to fix, and when tuned right and set up properly, can be as good as injection off road. some might complain about angles and so on, but the factory zook carby's are made for that. no power house by any means.
EFI would be the better choice. a computer to run everything, no off road dramas, good response and power for off roading, but if you drown the computer, ur stuffed. the factory computer for the G16b 16V can handle a small toyota supercharger if you really want more go.
you are asking which suzuki to mod..... what are your choices? sierra? jimny? vitara? i'm assuming you're in aus too, like me.