I'll see if i can help answer some of your questions.
1. Do you have to set PXE Options in DHCP to point to the PXE Representative?
If you setup a PXE rep in the same subnet, then you shouldn't need to configure anything. When you deploy the PXE rep it has it's own little DHCP client that will handle the requests. Now if you want your PXE rep to cross subnets then you would need to configure DHCP and setup an IP helper.
2. Can you set PXE to Automatically boot to WinPE only if a job is being pushed down? Can this be unattended on the client side?
There are 2 options in your PXE menu for your type of situation. If you want it to be manual, you would choose only WINPE Menu which will launch a OSD Menu to choose what you want to do. You control the menu listing on the server side. Alternatively you can schedule the OSD job from the server and it will pick up the job autmatically and process it self via WINPE Managed.
3. Does PXE Representative have to be a workstation OS? I believe the requirements are Windows XP and above. The PXE utility is windows based so im afraid you will have to be setup with a workstation with winxp or higher.
For question 3, the PXE Rep can be installed onto any Windows based system/OS from XP forward. This does include servers as well as desktops, laptops, workstations, etc. Hell, you could even have a VM run it. All it needs is to be on the subnet that you're looking to be able to WinPE boot systems on. You'll want the LANDesk agent installed onto the PXE rep in order to push the PXE Rep software to it...
Also, welcome to the good side (switching from altiris to LANDesk is always a happy move)...
One thing to note, try to attend the user group meetings if they are close enough to where you are. You can get a ton of information, and help, there. There have been several LDUG's where people have commented about switching from altiris to LANDesk and how much easier life became once that was completed. From my own [limited] exposure to altiris (at a previous employer) it's not nearly as strong a product. Plus, with Symantec now owning it, it's only a matter of time before it's morphed into something completely different. Imaging with the ImageW2 tool is a wonderful thing too... It's liceses are included with the LDMS product, so you won't need to use ghost anymore...
For answer two, there is also the PXE Holding Queue.
And also, WinPE Managed is the default if an OSD job is targeted at a device and that job is in the failed state.