I have TP-Link MR3020 v1 laying around and I was thinking to use it as a VPN server for only 1 client to watch Netflix/Youtube (just 1080p). Do you think it's possible since this model is really old. If yes, which build should I use? Currently it's on Chaos Calmer 15.05.1.
That depends on the VPN and on the bandwidth you require. It should work fine with WireGuard up to about 30-40 Mb/s. Any other VPN protocol will probably be unusable, I'm afraid.
EDIT: Ouch. That's a 4/32 device. You probably need to build OpenWrt from scratch, since no recent builds are being made for those devices.
You'd probably better get a new device with sufficient resources (Flash/RAM).
My take... try it. Don't expect much, though.
My recommended path:
Start with a custom image of 17.01.7 that omits LuCI and includes the extroot related packages (optionally also include preconfigured networking). Perform the extroot, configure networking (if not included in your image), and then install LuCI and WireGuard. Configure and see what happens. The device is not fast and it is low on resources, but I think it should be able to handle one client with modest bandwidth needs.
EDIT: It should go without saying that a newer device would be better in almost every respect. And I'm only recommending 17.01 because it works well on that device... but be aware that there may be limitations/security vulnerabilities associated with this old version, so I am assuming that it will sit behind your main router with a port-forward setup to direct your inbound VPN traffic accordingly (and in this context, I don't think there are any serious security concerns, provided you setup and secure that device properly).