I would recommend that you invest a little bit more than 100$ for the router or try to get a refurbished or used one.
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The Linksys WRT3200ACM is a great router (I'm using it myself) and it has a good performance at OpenVPN (1.8GHz dualcore) and also good OpenWRT support.
There are community builds provided by davidc502 which are ideal for beginners (Adblock, SQM-QoS, OpenVPN and a lot of other usefull things are allready pre installed)
Thread: https://forum.openwrt.org/t/davidc502-wrt1200ac-wrt1900acx-wrt3200acm-wrt32x-builds/ -
The Netgear R7800 is also a pretty nice router, should provide the same VPN peformance (1.7GHz Dualcore) and has afaik better Wifi Performance than the WRT3200ACM.
Community builds are provided by hnyman.
Thread: https://forum.openwrt.org/t/build-for-netgear-r7800/
and what about the modem?
Cable or xDSL? So what kind of Internet connection do you have?