Will I be able to upgrade to the newest macOS without any problem?
No: There is always a chance that the kexts & drivers will be outdated for a newer OS. So don't take the chance. If you must upgrade say from Mojave to Catalina then SAVE all of your data and do a fresh install of Catalina. If you system is running good, then stay with that OS. :)
What about Wifi/Bluetooth?
A great pcie card that is natively supported in macOS is the Fenvi FV-HB1200
It supports Handoff, Continuity, Wi-Fi, Bluetooth, AirDrop, and Instant Hotspot.