The MT300A is probably your best choice because of it’s robust memory and CPU. All of these devices have about the same capabilities in the software. The MT300a has an internal SD card slot as well as the USB capability, so can store more data. You can create a private LAN for sharing media on the AP side as well as a VPN tunnel on the WAN side.

Your goals may not be as easily realized as you hope however. You face 2 basic challenges which are unrelated to the hardware.

1 - AP connection speed. At least in the US most "hotspots’ are under 5Mbps and many closer to 2Mbps before VPN encryption. I find one needs closer to 7 for a good reliable video stream (YMMV). This means you probably need closer to 10Mbps before encryption.

2 - As I understand it, if you are trying to use iPlayer, the BBC is attempting to block VPN services for this. You may be able to circumvent this by using an “unknown” home VPN server.

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Please post back your experience