The VHDL of the PDP11 core is not vendor specific. I regularly use both Xilinx and Altera FPGA’s, and the VHDL works unchanged on both. Both Xilinx and Altera have cheap development boards that they promote for educational use – if you are a student, you can get a discount on top of the already low price. Most of these boards are below USD 150 even without the student discount, and perfect to run a PDP11 on.
The Altera FPGA boards I use are made by Terasic; these are very beautifully engineered boards that mostly contain everything necessary to run a simple PDP11 system on, even including SD card slots for the disk controllers. Most Terasic boards have expansion ports in the form of 40-pin headers – lots of pins to connect extra things to, but there are only a few accessory boards for sale, and most of those are not really appropriate for PDP11 systems.
My Xilinx boards are made by Digilent, and are maybe a bit more basic in their finishing. To make up for that, they have expansion ports for Digilent’s PMOD expansion modules; you can add the tiny PMODs as you require them, and in any combination you like. This makes the Digilent boards very flexible.
Which vendor you like best is very much dependent on your personal views. When choosing, it also makes sense to consider the design tools. Both Altera’s Quartus and Xilinx’ ISE have their quirks; these are really big chunks of very complicated software that will take some time to learn to work with. Neither is flawless, and both have some major flaws and errors in them that can really be aggravating at times.