I lifted this from a YouTube post, no idea if it's accurate:
Newer generations come with hundreds of improved and upgraded parts like 12k half shafts, differential coolers, stronger idler and pitman arms, ext. which may need to be replaced or will wear out quicker right out of auction if its an older model (early 2000's and before, or high mileage on gen 3 and below). Most people think all HMMWV's are the same which is not the case.
Gen 1: M998/M1097/M1045 ext. (1984-1993) 6.2L, 3 speed, most common components will need replacing (idler, pitman arm, ball joints, tires, bearings, ext.) They've likely sat for decades neglected in the elements. Can get them privately owned from $12k+
Gen 2: A1 series (M1097A1,ext)(1992-1993) Not many produced. Some improvements from Gen 1. Gen 3: A2 series M1045A2, M1123 ext. (1998-2005) MOST COMMON ON GOV PLANET (M1123)
Gen 3 is the absolute bare minimum you should by unless they are giving it away. The 6.5L and 4L80E Transmission are critical and will cost you more to install aftermarket than included in the purchase price difference between an A0,A1 and the A2. The 4L80E is a must have and the cost of parts and install is around $7,000 alone. The 6.2 to 6.5 engine is basically no difference. 6.2 reliability is reported to be better, actually. Basically the same engine with minimal torque and HP increase.
Gen 4: ECV Series M1167, M1165, M1151, M1152. (This is preferred) (2006 - 2011). Easily identified by the extended nose, 80mph spedo, factory A/C, tachometer, Wilwood brakes ext. First to feature the game changing 6.5L Turbo engine. This is worth some $$ (around $5,000 - $8,000 for parts and install. and it TAKES OFF. Massive upgrade and makes on-road drivability much more feasible and enjoyable.), factory AC (another $2500 - $3000), many upgraded drive train components, 12k half shafts, much better cooling, LED dash lights, improved everything basically. The M1165's commonly come with hard tops (with built in turret support and delete cover, making adding a turret a drop in, no need for the rare 10xx b-pillar turret support) and the iconic hard curtain. Add X-doors (if not included) and you're set. If you want a hard top, want to keep it cool inside, get less wet (yes, you will always get some water), or want a turret, this is the way to go: $7,000 4L80E $3,000 AC $2,500 Hard Top $1,000 Rear Curtain That's an extra $13,500 right there. And it wont be the correct roof or back for an ECV. Impossible to get your money back out of it. Decide what you want before you buy. Cheaper to buy with correct add-on's at auction.
Gen 5: REV Series M1167A1, M1165A1, M1151A1, M1152A1 (2007+ REV-B Serial Numbers 300,xxx+) Best HMMWV AM General has to offer. Reliability Enhanced Vehicle Everything the ECV series has and a massive upgrade to cooling (now drive train geared fan drive, not hydraulic), cooled differentials, upgrade to drive train and wear components, 24 bolt rims factory stock, parking brake enhancements, air lift brackets, ext.
