Hmmm. I really don't know to be honest with you. I am currently using a 812/813 version. Some say, the 813 is the best as others have put it. I use the 812 with Ka guard off and like the performance better. Tried ka guard off with the 813 but it just drives me nuts due to excessive falsing. The 812 does false but is bearable - you get better sensitivity but less falsing than the 813. These are the pre POP units and the versions up will be with POP. The other draw back with POP, is that, the "J" alert feature and I call it "J"ust be alert feature is that, it suppresses a real radar source. That's the main reason why I use a pre POP unit with no "J" alert to avoid supression. I had the 826 with POP and sold it because of the "J" issue.
The other thing to consider is, there will be manufacturer variance. Just because you have the same version number doesn't mean they have the same radar detection performance. There will some slight differences. Other factors involve is how you take care of the V1. Exposure to too much heat will degrade performance.
So which used V1 to buy? I would refer you to www.guysoflidar.com
and check there test data's for reference. Most V1's have very good performance. Just make sure you have it tuned up if you get a used one before you use it.