Questions about the kits.

Both kits have composite inputs and should be able to perform the task you want. As to know which one is better, it's difficult to answer. The EP3C25 is a newer generation than the EP2C70, and is probably a bit faster, but has less logic elements, leaving less space for your application. On the other hand if you just plan to put a Nios in there, both will support it well. The Nios is just an IP that can be placed in the FPGA and both the Cyclone II and III are supported.

You can't directly apply the design examples for the Nios II Embedded Evaluation Kit, Cyclone III Edition on DE-70 kit, as the hardware is different. It will need some adaptation. If you want to design something close to one of the examples for the Nios II Embedded Evaluation Kit, Cyclone III Edition, I'd suggest that you get that kit instead of the DE-70, you'll save yourself some time.

For the software side, you can use C or C++, but not Matlab code (at least not directly).

You don't need to buy a Cyclone III starter kit, the Nios kit includes everything you need to develop on it.

I'm not aware of any RF add-ons for the FPGA kits, and for the Wifi connection, as suggested in another thread connecting a Wifi access point to the Ethernet port on the kit will probably be the best solution.

I hope I answered most of your questions