Not for $20 shipped, but with the -for me - cheap NAS case the NanoPi Neo2 LTS is a option.
The Neo2 is available with 512MB or 1GB of Ram and works fine inside the NAS-Case
(I got 2 of these combinations running over a year).
armbian-support for the Neo2 (H5-CPU) is fine. HDMI isnt included, but with the NAS-case he has 2 USB 2.0-Ports, GBE,
a power-switch AND a proper Power-connector for a normal 12V power-supply
I am using the "original" Orange Pi cases.
For the smaller Zero-Board the black-cube while adding the Audio-/USB-expansion PCB.
For the "bigger" boards I like the clear ones - but for being only plastic they arent cheap.
hi running armbian ( kernel 4.14 ) and ovpn 2.4.6 and both nanopi neo2 boards.
ovpn is configured with cipher AES-128-CBC and auth SHA256, following results can be seen:
top - 04:53:47 up 26 days, 11:42, 2 users, load average: 0.16, 0.16, 0.09
Tasks: 102 total, 2 running, 57 sleeping, 0 stopped, 0 zombie
%Cpu0 : 1.0 us, 1.0 sy, 0.0 ni, 98.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
%Cpu1 : 1.0 us, 2.0 sy, 0.0 ni, 97.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
%Cpu2 : 20.7 us, 30.1 sy, 0.0 ni, 43.5 id, 0.0 wa, 0.0 hi, 5.7 si, 0.0 st
%Cpu3 : 0.3 us, 0.3 sy, 0.0 ni, 99.0 id, 0.0 wa, 0.0 hi, 0.3 si, 0.0 st
KiB Mem : 494152 total, 128980 free, 92528 used, 272644 buff/cache
KiB Swap: 247072 total, 229664 free, 17408 used. 382416 avail Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
1191 root 20 0 10180 6132 5324 R 59.1 1.2 137:16.90 openvpn
and following with single thread " iperf3 -4 -V -c 192.168.10.2 -t 60 -b 0 -P 1 "
also did not tweak kernel settings too much atm - both ends read:
net.core.default_qdisc = fq_codel
net.core.netdev_max_backlog = 1024
net.core.rmem_max = 33554432
net.core.wmem_max = 33554432
net.ipv4.tcp_rmem = 4096 87380 33554432
net.ipv4.tcp_wmem = 4096 87380 33554432
net.ipv4.tcp_congestion_control = bbr # RETEST westwood OR cubic
net.ipv4.tcp_max_syn_backlog = 1024
net.ipv4.tcp_slow_start_after_idle = 0
net.ipv4.tcp_no_metrics_save = 0
net.ipv4.ip_local_port_range = 9000 65535
If you want a cheap and low wattage VPN consider H5 boards ( eg nanopi neo2 or orange pi zero plus2 ) that should handle proper TV streaming (15-20Mbit) over ovpn this is your option.
If speed will be most important consider other platforms, I'm currently looking in to the ASRock J4005B-ITX and should do 300Mbit ...
I can only reply to question 4 and based on my limited sample of one: I bought their power supply with mi orange pi pc and it's been working 24/7 for the last year and half. Crossing fingers....
Gigabit does not make sense. I'm afraid you will not get more than 20mbps out of your board. This is a very very optimistic expectation, 10 is a more realistic number. OpenVPN is secure but there is a significant overhead.
Having a hardware encryption can make your board slightly faster probably, but definitely will make it cooler.
You should also consider processor speed and whether the kernel supports HW crypto acceleration. I have tried openvpn both in an OrangePi+ 2e (Allwinner H3), and an Odroid XU4, and performance is an order of magnitude faster in the XU4.
H5 has ARMv8 crypto extensions, so just make sure you choose a kernel that supports them.
Bigger boards = better heat dispersion and (better) voltage control = less (over)heating, better (non microUSB) powering = better stability. H5 is noticable faster than H3 and has better software support than A64 => IMO Opi PC2 should fit your needs.