If I setup OpenVpn on a server and use a vpn client on the laptop to
connect through it, is it enough?
It's impossible to say since we don't know what you're required specifications are. Let's be safe and just say "no."
Can someone perform a MITM attack or otherwise eavesdrop on my
Yes. No. Maybe. It depends on the encryption and negotiation types of the VPN.
If someone on the local network port-scans my laptop, will the open
ports be accessible to him while I use the VPN tunnel?
Yes. A VPN connection merely allows your PC to have an added route to a different network, and all communication over that network be secured to one degree or another. Your PC is still just as visible on any network that it is physically connected to, either via a wired or wireless connection.
Is there anything else I should keep in mind?
Yes. That VPNs and the broader spectrum of network security are complex things that need to have much time given to the pursuit of in-depth knowledge.