The main things IMO are 8gb RAM minimum (at least expandable), and look for a good graphics card embedded with a lot of RAM as well (especially if you are aiming for unbiased render engines like Octane, Lux or Cycles, all of which can be exported from Poser). EDIT:...and an i3 or better.

If your not going to use a Monitor with it for the most part, you may need a bigger screen (17 in), and sacrifice (ease of) portability.

There are typically limited upgrade options once purchased, so you wants to go nice up front.

(...for example, you can get a Toshiba Satellite, i3, 8gb ram, 2gb Radeon 7670, for about $600...its a decent machine.  i5/12gb for $800.  i7/16gb for $1K)