I don't have any idea how long but it is near identical to the 2012 model, it runs 10.10 so unlikely to be killed off soon.
My Mac Pro 2006 is not getting security updates because I have only 10.7 on it and it has an older firnware than the newer models.
GPUs are easy though there is more to know than just dropping in any PC card so easy physically. That would be a whole other subject.
The processor can be upgraded. RAM and hard drives. And for the price, nice but it really depend what you expect to perform and the software and projects. You said you wanted a Mac so the cost of software etc is also a factor. Usually start from there.
Dual AMDs and Apple FinalCut Pro-X with 6 or 8-core and 32GB RAM along with storage, SSD(s), and "M.2" type SSD (1000MB/sec, the Mac Pro is almost unique in easy support for SSD blade storage to use and boot from).