STAND or SHELF: Most plotters come with a wheeled stand, but some don't have the stand and are designed to sit on a table or shelf. A wheeled stand will cost you more AND take up more office floor space, but they are easy to move. Designjets are also very heavy - the bigger models up to 180 pounds - so you will need a very solid shelf to place it on AND the shelf has to be low enough to be able to open the window and peer inside...so it can't be higher than eye level.
COLOR or MONOCHROME: Are the documents you will print going to be in color or black and white (monochrome)? If most of your documents are black and white, why pay extra for color? That said, all color machines can be set to JUST print in black and white to save the cost of color ink.
INK: Can you find the ink for the plotter in local office supply stores or will you have to search the internet and buy outdated ink? Can you have the cartridges refilled?
NETWORKED: Many Designjets will have a Jet Direct card in the and this is a bonus as you will be able to easily connect your plotter to the network in your office.
DRIVERS: What operating system are you using? Will you be able to find a driver for the plotter that will be compatible with your operating system? Note: Most Designjets will work well with Windows 7 (64-bit) by using the 64-Bit Vista driver that HP has on their site.
CONNECTIONS: How will you connect your computer to the plotter? The Designjet has a parallel connection, but many newer computers don't have a parallel connection. You can overcome this obstacle by buying a USB to Parallel Cable.