I'd definitely start with a Altera starter kit. Comes with all the tools, documentation and software.
http://www.altera.com/products/devkits/altera/kit-terasic-cyclone-v-gx-starter.html
My questions is : if you write a code for a specific FPGA, is it compatible for use in another FPGA, let's say from a Cyclone III to a Cyclone V ?
Just as a note, that kit (I got it too) doesn't come with the CD that includes the documentation and software, but you can download that from Terasic after registration (no need to buy the kit to register). Also you have to install QuartusII from Altera.
As for porting to other FPGAs, if you stick with Altera they have a device migration built in in QuartusII. Also the Terasic boards come with a configuration utility that creates (only Verilog) templates and sets the pins that you are planning to use via a nice GUI.
A lot of their examples use the Qsys system integration, it sure makes life easier, although I kind of missed the simplicity of SOPC Builder that they used before, but I'm not sure how easy it will be to port to say Xilinx once you use those tools.
I did write a review here:
https://www.eevblog.com/forum/microcontrollers/terasic-cyclone-v-gx-starter-kit-(c5g)-review/Another caveat is that Terasic does favor Verilog on their examples. But converting the generated templates to VHDL is easy, also you can import Verilog modules into VHDL code, a bit tricky but not hard at all, or just stick with Verilog.
All the datasheets for everything is included and you can even get more (specially for the HDMI TX chip) from the Analog Devices website.
I did look at the Mojo when I was looking for an FPGA dev board, only reason I didn't is that they didn't have a lot of source, examples etc compared to what Altera/Terasic offers.
Papilio, that is a different thing, it has a lot of community support and pretty cool projects and it's pretty affordable. No HDMI however.
I believe the Composite Video is a 7 bits R2R implemented DAC someone made that gives you 64 colors (good for 8bit graphics). Their mega wing supports VGA output with 4 bits R2R DACs per color for a 12 bit total, or 4096 different colors.
But the cool thing about FPGAs is that anyone could implement an "HDMI" wing like this guy did (I quoted HDMI because it's not really HDMI, just LVDS driving the LCD directly, but digital nevertheless):
http://forum.gadgetfactory.net/index.php?/page/articles.html/_/papilio/papilio-wings/alex-makes-hdmi-wings-for-the-papilio-plus-r61But nothing stops anyone for using the ADV7513 HDMI-TX chip that is in the Cyclone V GX starter kit for real HDMI, and maybe do a better implementation since the starter kit didn't hook the HDMI audio part to the FPGA since it has an audio DAC already, but the pins for HDMI audio are there but unpopulated so you could add a header and use that instead.