Big dad has said that when all costs are considered, they are about a wash. I usually say keep the stock stuff if you are doing a mild build, but in your case, I think the MII might be the better way.

MII
+ plenty of options with ride height track width, air bags and readily available components and kits. Also has rack and pinion factory engineered from the start. Able to beef up the proven design to handle current speeds and weight.
- having to do major frame work and proper welding. First buy-in cost could be higher if going with a full kit with lots of options, but this can be spread out too.

Stock with dropped uprights, shock mount, disc conversion, rack and pinion swap, and air bag conversion
+ no frame work, only welding for the shock mounts maybe the steering, and maybe the lower control arms for the air bags and lowering, No sheetmetal fitting after.

- old technology never designed for 75+ MPH, old harder to find suspension parts, stuck with track width and frame height, all the mods come from different sources so little to no support, spending almost as much or the same for less handling, and probably less resale value. More restricted fitting, since the MII frame stubs can be designed for a bigger radiator, easier exhaust routing and starter access.