Speed is not necessarily impacted much with a quality VPN that uses Wireguard and cost is minimal.
Using syncthing with a VPN is not a problem as long as the VPN provides split-tunneling, such as IVPN does.
Whether a VPN would benefit you is another matter altogether that depends on what you're attempting to achieve. Lastly, know that the vast majority of VPN's are garbage regarding privacy. Out of the possibly hundreds of VPN's out there, the only ones that seem to take privacy seriously are Mullvad, Azire and IVPN. Most of the remainder are owned by a small handful of very sketchy companies.