i'm far from an expert, but i would emphasize a couple things from cobrabrat's list. i'm assuming you are considering a GL purchase.
IF AT ALL POSSIBLE, PERSONALLY INSPECT THE GEN BEFORE BIDDING!!! i would not consider buying one based on the pictures GL posts!
when i bought mine, GL had probably 20 for sale at the same time/site. i went and looked every one over, taking note of deficiencies for each serial number. most had SOME issues. many of them were fairly minor, but they all add up.
in particular, you want to examine the fuel system as best you can. specifically - open the tank and look inside with a flashlight. rusty? could be a MAJOR PITA to rectify. i wasn't willing to deal with that problem, so ANY significant rust and i ruled that unit out. not sure how you could examine/test the glow plugs/intake heater.
are the fuel filter cans (holders / whatever you want to call them) all there? is the engine oil filter can/holder there? the units my father and i ended up buying were missing the spring inside the oil filter can. surprisingly expensive parts to buy from delks ($30 or something insane).
is the injection pump actually there? if not, again, i was not willing to source one (BIG $$) and then install/time it. see the many threads on needing timing buttons. are the fuel hoses in place and in decent shape?
does the throttle linkage move easily? if not, fixing / replacing it probably isn't a huge deal, but it could be indicative of a IP problem - see above.
many/most(?) are trailer mounted. are you going to tow it home? better bring a small compressor as the tires will likely be flat. are the tires in decent shape or safe enough to tow home on? do you have the necessary towing pieces? a pintle hitch and some way to get brake lights? i bought an el cheapo trailer light kit from harbor freight with magnetic brake/running lights. pintle hitch height may be an issue depending on tow vehicle. my 003 was perfect hooked up to a straight draw bar on my modern dually. a compact pickup or small SUV will probably be quite low.
is the exhaust taped closed? don't want water in there...
pull the engine oil dipstick - anything on it? all of the GL units i've seen were emptied, but the units we bought had a trace of oil on the end of the dipstick. some clearly had WATER in them!
are the battery terminals/wires visible and in reasonable shape? is the 'jumper' from one batt to the other there?
some folks have gotten the ground rod that is supposed to go with the units, but it seems like most don't have them (mine did not). i think there was also supposed to be a hose to run from the aux fuel input to a drum, but again, mine did not have it.
are the gauges fogged up? is the hour meter way newer than the rest of them (ie probably shouldn't be trusted).
is there a rebuild plate above the original serial number plate? it seems a good portion of the units being auctioned off were taken out of the field, rebuilt, then put in a warehouse. the two we got showed somewhere around 2(!) hours on them, which of course, seems impossible. but they were rebuilt in 2008 (tier 2 reset), and appear to actually be nearly new in almost all respects.
i think you have to go in eyes open; that is, there are quite a few things you simply cannot check in a GL site. there is risk involved. people have gotten some with cracked heads, cracked blocks, dead gen heads, blown VR, etc. a good portion of them need filters, batteries, engine oil, and diesel, and they fire right up. on mine, the battery charging VR was dead. on my dad's, the oil pressure sender was bad. if you simply can't stomach the potential for buying a dud, buy one that has been gone through and tested from sewerzuk or another private party.
ahm