I assume you are asking about options for a T7500. If so, you should get at least 8GB of memory. Have you looked at Dell's Outlet (dell.com/outlet)? You can get a very nice T7500 for a fraction of the cost of a new one WITH a warranty.
For the 3 monitor solution, you may have to add a second GPU. All of the options I saw via Dell's Outlet website were for dual monitor GPUs. Adding a second, or purchasing a card capable of more than 2 displays is an option.