I know nothing about them! So is a 404 a bad thing?
Not a bad thing, but many buyers have unrealistic expectations of how usable they will be.
The 404 is petrol powered, with a 2.2 liter 6 cylinder engine that makes about 80 horsepower. Because of their gearing, they perform well off-road, but they are pretty overmatched for cruising North American highways. You can go faster than 55 mph but the engine will be wrapped very tight; 45 to 50 mph is a 404's most reasonable cruise speed.
Add to that that they their military roots meant they had few creature comforts. Their cockpits are tight for driver and passenger and they are noisy. Also, at anywhere from 10-14 mpg, fuel economy is not stellar.
The one on eBay has obviously had modifications which might mitigate some of the "shortcomings" . . . if you consider it that way. The 404 is tough to beat for certain uses, but normal highway use is not one of them and their prices reflect that. Decent to good 404s generally fall into the $5 to $10K USD range. With the modifications, the asking price for this one is probably not out of line. It'll be good, though, if the buyer knows something of how they drive before doing the deal.
BTW, there are good overviews of the Mog models, including the 404s, through this website:
http://www.unimogcentre.com/.