Lenovo makes quality workstations - I would stick with the brand you have accessories for. I prefer the T or X series but work with your budget. Unfortunately, you're going to get strong-armed into Win8.1 unless you wait for August when they're releasing them with Win10.
May I suggest you simply plan on replacing the hard drives with a ssd.
I have done this on all my laptops and the performance improvement is remarkable.
Pick one test candidate.
Buy a Samsung evo large enough to hold the current contents plus some.
Samsung offers a ssd migration tool.
Connect the ssd via a usb cable and you can clone the original to the new.
Then just replace the old hard drive with the new ssd.
And... keep the old hard drive as an emergency backup.
If they're having issues with opening multiple spreadsheets simultaneously or overall sluggishness when when multiple programs are open, then checking to see if it will support 8GB of RAM would be the best upgrade.
If they're upset by how long it takes to load windows or to launch a file, then a solid state would likely be the best upgrade.
If they're running a data analytics program and it's taking forever to process, then it's time to look at a new generation of laptop for a faster processor.
Or, as many of my execs like to do, they may just want something shiny and new to make them look special in a meeting.