The significant factors are 1) minimize the leakage, 2) use a pump with enough capacity to pump the system down quickly and overcome leakage but still hold the work piece and, 3) prevent contaminants (coolant and swarf) from getting into your pump.
Here's an inexpensive pump that you could try for low $$.
New Vacuum Pump 1 Stage 3 0 CFM 1 4HP Rotary Vane Deep HVAC Tool AC R410A R134 | eBay
Regarding leakage, a lot depends on the specific configuration of your work piece and what you're trying to accomplish. Use neoprene sheets to cover any holes that your work piece doesn't. Your work piece must mate precisely with the vacuum table to minimize leakage. Can you use a very thin elastomer gasket (with holes) to seal the work piece?
To prevent contaminants from reaching the pump, get a pressure pot (at least 1 gallon) and plumb it so any contaminants go to the bottom of the pot and the vacuum side fitting is at the top of the pot (drawing clean air only, zero coolant). The pressure pot must be easily cleanable and have a visible level indicator (glass jar, sight glass, ???) so you know when to empty it. Put a secondary (dry)filter between the pot and the pump. Contaminants will destroy a pump quick so the filtering is of prime importance in a machine shop environment. If you're not going to use coolant/cutting oil, the danger is less so the filtering can be simplified.
You may want to set up a valve system that isolates the chuck (table) from the vacuum side before venting the chuck. That will allow the filter system and most of the plumbing to remain at low pressure while changing the work piece. You won't have to wait for the whole system to pump down from atmosphere.
Plumb in a vacuum gage close before the chuck so you know when it's ready to use and at the pump (and other places in your plumbing, depending on complexity) to help find leaks.
You probably see now that the filtering is a bigger deal ($$) than the pump and the table.
EDIT: Thanks to Sami's link, here's the section that includes filtering:
http://www.gastmfg.com/vphb/vphb_s5.pdf