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If you are going with the Slash 4x4, I highly recommend you go with the Slash 4x4 Ultimate. It has the best value compared to other Slash 4x4 trims since it comes with many upgrades (LCG chassis, aluminum bearing adapter, aluminum caster blocks, aluminum steering blocks, aluminum rear hub carriers, GTR shocks, sway bars, softer compound tires, telemetry sensors, Bluetooth module, etc.) In total, you will save roughly $150+ in terms of parts if you go with the Ultimate.
As for you Slash 2wd, turning it into a Drag Slash is an awesome idea! You can probably get inspiration from Traxxas, since they recently revealed their Drag Slash (legitimate model for sale, not just a project), with the Magnum 272R transmission (sealed differential, aluminum motor plate, upgraded slipper clutch, and redesigned input shaft. Of course, it's not released yet, but it will in the coming weeks, so I think it's worth the wait:
https://traxxas.com/drag-slash-new-transmission