Hoff is a cynic who makes bleak comments about humanity's chances of survival. Meanwhile, he knows that no buyer will stop purchasing from him, because they either need the medicine or are addicted to the chemicals. He likes to think that, in his own way, he's helping to ease humanity's suffering in its dying years.
Doc Hoff appears outside of frequent stops every time you fast travel to that location, go through a loading screen to that location, or wait at the site for a couple of hours. A good way to find him is to fast travel to Rivet City and go down the metal stairs. If he isn't there, wait for one or two hours so a merchant spawns. If the merchant isn't Doc Hoff, wait for one or two hours again. Repeat until you get him to spawn.
Merchant Empire: After talking to Uncle Roe about caravan merchant investment, his repair skill jumps to 65 (allowing him to repair items to a max of 79%). His inventory (and the number of available caps) also improves, and after the second investment, he rewards you with 5-7 stimpaks, depending on your level.
pcps3xbox360 Doc Hoff can appear at the Jefferson Memorial during the time of the quest "Take It Back". He is almost always instantly killed by Liberty Prime, and his body can be very difficult to find with all the dead Enclave soldiers. Fortunately, his corpse will take more time to disappear, making him easier to locate if you wait about five to seven days after you finish the mission. [verified]
ps3xbox360 When starting a new game, it is possible that Doc Hoff will spawn without his pack brahmin and caravan guard. [verified]
pc Sometimes Doc Hoff and his guard will be separated from his pack brahmin and make rounds separately. One way to fix this is by finding the separated brahmin and using two console commands to move Doc and his guard to your position near the brahmin: moveto.player 63c06 (and) moveto.player 522bc1. [verified]