Here is a brief synopsis of the differences among models.
There is no internal differences among lowers or uppers, the V# correlates to the load out. Some models the rail, and rail length are the only differences from another. Most models come in more than one cal, the V3, V5, V7 are offered in 5.56, 6.8, 300 aac. (MK18 or SBR is 5.56 or 300)
MK18 is a 10.3" barrel, CARBINE length gas tube, flat top with low pro gas block and RISII MK18 handguards around 5.6lbs -6 lbs (300 aac vs 556) 300 AAC model is not technically called a MK18, it is the "DDM 300 SBR" and it runs a Pistol length gas system.
V1 is a carbine length gas system with 16" M4 profile barrel, Front Sight Block, Fixed rear site, and a 12" DDM4 rail (6.15 lbs)
V2 - nearly identical to V1 but with a 7" rail, designed to be a lower weight option to the V1 (6.10 lbs)
V3 - its a V1 with a mid length gas system, Govt prof barrel and a 9" rail (6.12 lbs)
V4 16" M4 barrel with a low pro gas block and a 9" Monolithic DDM4 rail (flat top, designed for aftermarket optics, no front sight post, no irons come with it) ( 6.7 lbs)
V5 16" Govt prof barrel with low pro gas block and a Monolithic DDM4 rail 12" (V5 is designed as a aftermarket optics ready platform, no irons or sights come with it) (6.4lbs)
V7 16" M4 barrel with low pro gas block and a modular FF tube rail, 12" (no front sight post, designed for aftermarket optics) (6.9 lbs)
V7 SPCII - 18" CMVS barrel in 6.8 the rest is the same as the regular V7.
Several models are offered with a light weight barrel option, and more than one barrel profile. The V3, V5 and V7 are offered in more than one gas system length. Some models later came with revised rail systems, my last DDM4V5 came with the newer style rail system.
Long story short, if you are trying to decide what one to pic ask yourself 1) am I going to put an optic on it ? 2) do I need irons? 3) how much rail space do I need? 4) do I care how heavy it is? you can narrow it down fast.
Most people take the V5 because its light, its optic ready and they intend to run a aimpoint/eotech or etc and because it is a gov profile barrel.