You'll probably only find c7 v5 these days (afaik with non-detachable antennas), but you can find faster/ better ipq40xx devices (e.g. AVM Fritz!Box 4040 or ZyXEL NBG6617) for around the same amount of money (slightly cheaper).
ipq40xx is an ARMv7/ cortex A7 based SOC family (IPQ4018, IPQ4019, IPQ4028, IPQ4029), which is newer and faster (four >=638 MHz cores instead of a single 750 MHz core, newer wave2 wlan) than the ar71xx/ ath79 mips_24kc SOC used in the Archer c7. The particular devices I've suggested are available, supported by OpenWrt, relatively easy to flash and have enough flash/ RAM (32 MB flash/ 256 MB RAM; you really need >=256 MB RAM for good performance with ath10k).
Aside from detachable antennas, the suggested ipq40xx devices meet and supersede those requirements (and especially for VPN uses they will be better than the archer c7).
Cross reference with https://wikidevi.com/ - do read the flashing instructions and hints about different hardware revisions sporting different hardware before and discard devices with 128 MB RAM or less.
As the Archer C7 goes, do not buy a V3 it is not supported. V2 is OK but of course will only be found secondhand. In the V2 V4 or V5 you could buy a foreign model and replace the power supply with one that fits your wall outlet. The power input to the V4 is 12 volts DC with a very standard 5.5/2.1 mm plug. This is likely also the case with the V5.
I think they shipped with a 2.5 amp supply. Power supply with more amps will be OK, it will just be underutilized. If you aren't plugging anything heavy into the USB ports, 1.0 A is enough.