I think it’s quite a bit of work to do that upgrade. (good 3->5 upgrade thread here). And $500 sounds like a lot for a Taz 2. I bet you could get $100 off that easy. Would I do this upgrade with a $500 Taz 2? Not a chance. You’re spending signifiant money and time to upgrade a 7 year old printer to a 3+ year old design, in a product category where product innovation is driving rapid technological change. But that’s me.
All of the production files being open source and available really helps if you choose to do this, but if you do go for an upgrade, do it for the adventure. If you just want to print, print well and print consistently, stick with the Mk3. Or you can find good deals on Taz 5s and 6s, and also Minis. (which are underrated, but are awesome for their fantastic frames and smaller build plates, which lets them print a lot faster than a taz for things like part prototyping.) Used lulzbots are starting to sell at a discount, which does make them more attractive. Since all of the documentation is online, they are future-proof from a modding and maintenance standpoint. This is a huge and hugely-undersold value. (and also applicable to Prusa)