can i upgrade my late 2012 imac 27 inch to ssd or fusion drive?

Not easily and probably not by yourself. Disassembly of the new iMacs (late 2012) is very difficult and not to be done without good technical understanding and extremely careful work. The screen is glued in place and has to be removed before any other disassembly can be done.

A shop with the appropriate technical expertise would be a better choice rather than attempting it yourself. However, even doing it this way will probably void the warranty/AppleCare. The internal drive is not a user-replaceable part in this crop of iMacs.

imho I would max the RAM before even thinking about replacing the hard drive in one of these.

If you're beyond the return period for the iMac and you insist on having an SSD or Fusion drive, then I'd suggest you sell your existing iMac and buy a new one in the configuration you now wish. As Martin said, it's really difficult to upgrade your iMac. Doing so yourself will absolutely void the warranty, and paying an authorized service center to do the upgrade probably won't be much if any less than what you'd lose in selling your current iMac.

Alternatively, you could look into a Thunderbolt-equipped external SSD. I haven't seen benchmarks on how an external drive would compare with an internal SSD, but it's something you could investigate, anyway.

Regards.