I think what Terry was saying was add a little more detail to the application. PLC's come with a multitude of options and features you may want to look at some of the more common brands like
http://www.ab.com
http://www.automationdirect.com
http://www.gefanuc.com
http://www.sea.siemens.com
There are other brands too.
Specifically can you state a possible specific number of I/O both analog and digital that would be needed.
What will be the conditions the machinery works in?
As was stated it isnt hard to find a plc works with 24vdc, I think an earlier thread asked once about 12vdc plc's but I dont remember anyone knowing of any. If the system is small there are a multitude of "brick" style plc's that will do what you are asking. Look at Automation Directs DL06, its a little dynamo that even offers an LCD interface ($59) that allows you to change/view settings and display alarms etc. I am redoing a machine with 4 to 5 electromechanical timers with just one of these and havent scratched its memory capabilities.
Anyway this is R&D correct, one of the first step is to determine what you need specifically in the way of I/O and communication connections for an Operator Interface. The other step is too determine conditions it will work it to determine what codes/guidelines must be met...ie is it outdoors, hazardous conditions, desert)hot climate), near/on water (wet conditions), cold etc.? This can determine what type cabinet it must be housed in, what ratings must be obtained. Are you concerned with use in the US only or will it need to be rated for other countries or standards?
If you can give the details as mentioned above these guys can offer more specific advice on what could work for you. Naturally all they can do is offer advice on possibilities, the choice of what to use will be yours. The system sounds small and shouldnt be difficult to fit with a plc to do what you need/want but we dont know all the details yet.