Hello Spidey,
Have you updated the package list first?
You can also find the desired package in the list of available packages as soon as you refresh the list.
CPU-wise they are the same. RAM and storage are double in the MT300, so it will server longer time, provided you are content with the speeds you'll achieve.
Try it again today at some point. If you didn't understand from the other topic that @tmomas posted, most likely something went wrong during the compiling process, hence the missing packet. It should be available after the next building cycle. If it is not, we'll notify the maintainers to have a look at it.
I get the same error, seems to me that the mirrors of OpenWRT are damaged.
root@OpenWrt:/etc/opkg# opkg update
Downloading http://mirror.kumi.systems/openwrt/releases/18.06.2/targets/ramips/mt7620/packages/Packages.gz
Updated list of available packages in /var/opkg-lists/openwrt_core
Downloading http://mirror.kumi.systems/openwrt/releases/18.06.2/targets/ramips/mt7620/packages/Packages.sig
Signature check passed.
Downloading http://mirror.kumi.systems/openwrt/releases/18.06.2/packages/mipsel_24kc/base/Packages.gz
Updated list of available packages in /var/opkg-lists/openwrt_base
Downloading http://mirror.kumi.systems/openwrt/releases/18.06.2/packages/mipsel_24kc/base/Packages.sig
Signature check passed.
Downloading http://mirror.kumi.systems/openwrt/releases/18.06.2/packages/mipsel_24kc/luci/Packages.gz
Updated list of available packages in /var/opkg-lists/openwrt_luci
Downloading http://mirror.kumi.systems/openwrt/releases/18.06.2/packages/mipsel_24kc/luci/Packages.sig
Signature check passed.
Downloading http://mirror.kumi.systems/openwrt/releases/18.06.2/packages/mipsel_24kc/packages/Packages.gz
Updated list of available packages in /var/opkg-lists/openwrt_packages
Downloading http://mirror.kumi.systems/openwrt/releases/18.06.2/packages/mipsel_24kc/packages/Packages.sig
Signature check passed.
Downloading http://mirror.kumi.systems/openwrt/releases/18.06.2/packages/mipsel_24kc/routing/Packages.gz
Updated list of available packages in /var/opkg-lists/openwrt_routing
Downloading http://mirror.kumi.systems/openwrt/releases/18.06.2/packages/mipsel_24kc/routing/Packages.sig
Signature check passed.
Downloading http://mirror.kumi.systems/openwrt/releases/18.06.2/packages/mipsel_24kc/telephony/Packages.gz
Updated list of available packages in /var/opkg-lists/openwrt_telephony
Downloading http://mirror.kumi.systems/openwrt/releases/18.06.2/packages/mipsel_24kc/telephony/Packages.sig
Signature check passed.
root@OpenWrt:/etc/opkg# opkg install openvpn
Unknown package 'openvpn'.
Collected errors:
* opkg_install_cmd: Cannot install package openvpn.
root@OpenWrt:/etc/opkg# opkg info openvpn
root@OpenWrt:/etc/opkg#