I would be looking for at least: 3-jaw chuck, 4-jaw chuck, steady rest, follower rest, tailstock drill chuck, tailstock centers (dead and live), toolpost and tool holders, some cutting tools. Quick change tool post and holders for it would be very helpful because the lantern-style post and holders are more difficult to deal with for the majority of operations. As I stated before, a used set of change gears for the Atlas will likely be close to $200 or more. They are available periodically but are not cheap.
Regarding your question about machining the camshaft: What is the diameter of the shaft? If it is more than 3/4" you will be limited to holding one end in a chuck (preferably a 4-jaw so it can be dialed accurately). The other end would need to be held in a steady rest to be supported well enough to drill it. You could drill half-way through, then dismount, flip end-for-end, remount and redial it at both ends (chuck and steady rest), then drill to the already-drilled hole. The other problem becomes length. You effectively have 30" of bed length available between centers. You have a workpiece that is 18" long, then a drill chuck that uses about 2-3", then a drill bit that needs to be 9.5-10" sticking out from the drill chuck. That totals 30-31" and uses all of the available bed length in a
best-case scenario. More likely, the chuck on the spindle will take more space than a center would, so that uses up another couple of inches that you don't have. Also, the tailstock ram on these machines only has about 2.5" of travel. That means that you cannot drill the whole length from one position. Another option would be to use a drill that would be mounted on the carriage. This would eliminate needing the tailstock for this project and it could be removed from the lathe. It would leave more room to work and would eliminate having to clamp, release, move, reposition, re-clamp the tailstock for drilling many times for one operation. There are some options, and it could be accomplished, but might not be ideal. I completely understand, however, the need and desire to obtain useable machine(s) on a tight budget.
The direct answer to the question
Why can I just put one end of the cam or bar stock in the chuck and have a tool rest to help?
is that without real support on the right end, it would be quite unsafe that far out from the chuck. That is the reason for needing a steady rest. If you were not drilling the end, the tailstock and center would support it, but you need to work the end so it requires a steady rest for support.
Sorry for the long post, but I hope it helps you some.