There are some minor disadvantages to using a dynamic IP. If someone who previously had the IP address you've been assigned did something nefarious on a service you use, it's possible that IP address might be banned. Usually, VPN providers are very careful about checking their IP addresses against blacklists, so the chances of this being a problem for you are slim.
Not all VPN services require that you pay. There are, in fact, many excellent free VPNs. But all of the free VPNs we've tested have some kind of limitation. Some limit you to just a few simultaneous connections or devices on an account. Others restrict you to a few hundred MBs of data per day or per month. Others limit you to just a handful of servers. Still others do all of the above.