iMac/Apple Purchase Advice

I have a 2012 iMac at work that has become very sluggish at times, is sometimes unusable with multiple office applications open, frequently is extremely slow to reboot with Sierra.

Recently, upon restarting the gray progress bar gets stuck either at 0 or 100% and just hangs there for 30 minutes, an hour, 2 hours, etc. This morning it took about 4 power cycles and a couple hours to get it to come to life.

First, any idea what is causing this? I've checked the drive health, tried to remove any startup items that could cause lag, disabled Siri, etc.

Second, employer is probably just going to buy me a new computer instead of opting for an SSD or additional RAM (presently at 8gb) first. If I go the iMac route, how would you prioritize purchasing?

For example, you can get discounts on prior year models ("introduced 2013" through 2015 range). Will I automatically be disadvantaging myself in getting one of those?

In addition to that, what about SSD vs. Fusion and the need to max out RAM on a new machine? I don't do anything particularly intensive, but I'd like to be able to run a bunch of inefficient items that rob some resources at the same time without it frequently seizing up on me (significant dropbox syncing, Word/Excel/OneNote/Outlook, Chrome)

Thanks for the help.