This sounds like a worthy project, and I hope it will develop into a resource we can point people to when they show up for the moment I will focus on advice regarding the first step which is identification and buying, first regarding GL.
In general GL auction descriptions can lack detail and often be incorrect on such things as model numbers, the good news is this seems to be due to lack of attention to detail on the part of the person listing the item, and is unlikely to be due to intentional deception. (this is not true of ebay auctions, etc. where people have been known to switch out cover panels, meters, etc to pass off 400 hz useless paperweights as useful 60 hz units), so the key here is make sure the description matches the actual item, also if you can inspect in person so much the better, as sometimes GL will list items with the wrong photos attached particularly when there are large lots of similar units listed.
Be aware that condition codes and hour meters don't always give you an indication of the condition of a generator, hour meters may show low hours because it never ran right, or the meter may have been changed out on a high hour unit when it stopped running. Condition codes are even worse, I would go so far as to say they are useless, I have bought several items listed as HX or scrap that were either fully functional or only needed minor repair (a good example of this is a lot of 4 HX code floor jacks I bought, 2- 4 ton and 2- 10 ton, 3 of the 4 were fully functional, the last one someone had twisted the steel release rod inside the handle into a corkscrew, how they applied enough leverage to do this I have no idea). The lesson to learn from this, is to carefully inspect photo's videos, and if possible the unit in person, look for missing parts, this can be signs of a dead unit that was used for parts, or at the very least some of these missing parts can be expensive or hard to locate.
Before you bid/buy make sure you read the fine print, where is the unit located, what are the access terms, does the site offer loading assistance, often you will see items at a remote site mixed in with the listing that are located at a manned GL site, remember if you do win you have very limited time to arrange for pickup. Also before you bid be aware you are buying AS-IS, and even if you get lucky and get a runner (based on the general experience around here these can account for about 50% of the good looking GL generators) you will still likely have to invest in fresh batteries, filters, oil, etc. Then there is that other 50% which may range from requiring minor work, on up to being good only for parts or a paperweight.
Next is a rundown on commonly seen generator models that are useful for general personal use / standby power, etc. I will limit this to the 1.5 - 30KW size range MEP series generators (All military KW ratiings are ultra conservative and are continuous load ratings in extreme environments, which should not be confused with the peak ratings seen on consumer generators) , if your looking for anything bigger than this you likely known what your doing, or you should. The MEP sereis is the standard military generator series adopted in the late 1960's when the DOD realized that logistic support for the vast number of models they had been buying prior to that was not practical.
First off the gasoline units (these have the potential of being converted to run on natural gas or propane)
These are all Viet Nam era designs and were designed when little thought was given to fuel economy
They are the
1.5KW MEP-015 -1.5KW 3600 rpm generator, basic simple, good for a couple of KW will keep the refrigerator and a few lights and fans running in a power outage as long as you can feed it.
3KW MEP-016A o MEP-016C - 3 KW 3600 rpm generator, can supply 1 or 3 phase power (120,240,120-208 3 phase) There is also the MEP-016b Diesel version of this unit powered by a 1 cylinder Onan built in the 1980's, as well and the MEP-016D and MEP-016E, the MEP-016D is an A or C gasoline uint that has been retrofitted with an L70 Yanmar Diesel as a life extension program (note many of these will retain their original labels and may be listed on GL as gasoline generators), the MEP-016E is a B series Onan Diesel that has been retrofitted with an L100 Yanmar Diesel. These yanmar powered units may represent the lowest cost of ownership military surplus generators as they are powered by current production model enginge and many parts are easily available. The MEP-016b is also available as the MEP-701a which is just a MEP-016b in an ASK housing
5KW MEP-017 3600 rpm gasoline generator, ( I don;t know much about them, but think they have the same 1 or 3 phase voltage options as the MEP-016 family), one thing is for sure their 4 cylinder military standard engines will drink lots of fuel.
10KW MEP-018 gasoline generator much like the others, very thrursty
Now for the Diesels:
3 KW MEP-016B MEP-016D, and MEP-016E (see mention above in the gasoline generator section)
5KW MEP-002a 1800 rpm diesel genrator powered by the venerable Onan J series 2 cylinder diesel engine, you will find these built by multple contractors, (Libby, Onan, etc.) all are built from the same parts and at this point after numerous rebuilds are likely mix-and match anyway. These also have reconnection switches to output 120V,240V or 120/208 3 phase. Sometimes they will show up for sale with theASK sound kit option installed, these tend to draw higher prices since all the MEP series diesel are notorious for being LOUD.
10KW MEP-003 Big brother of the MEP-002a except powed by a 4 cylinder Onan J series engine, same other comments apply, largest MEP series generator designed to offer single phase power
15KW MEP-004 requires wiring modification to run in single phase mode, generally considered to be too large for most personal / home standby use engines have a bit of a reputation for wet stacking when ran at light loads
30KW MEP-005 - see MEP-004 info.
Vintage units, there were numerous models built prior to the introduction of the MEP series in the late 1960's, as a general rule these should probably only be considered by those interested in vintage engines and equipment
The MB sereis, the MB sereis of generators were units built for the Air Force from the late 1960's through early 1980's in general avoid all of them, they had overly complicated control systems, and were designed like Air Force planes of the time to make them easy to service by aircraft mechanics using standard aircraft components, this makes parts for them very hard to get and very expensive.
Anything not listed above will likely be exotic or a relatively usesless 400 hz (only good for aircraft , missile and radar systems) or 28V battery charging APU generator.
Ike
and no