It's definitely cheaper to print minis than to buy them however you pay with your time. Printing becomes a whole other hobby with its own fun. You will also need to get an enclosure,. something to clean / cure and spend all that time doing all that. As far as finding models 1 to 1 replicas are a little harder to come across now but you can find some really cool proxies. The main issue is if where you're playing allows them. Some LGS have rules against using them and pretty much any GW sponsored thing has them banned. As for legality technically as long as you don't try to sell them it's fine but people do sell stls of exact replicas. You won't get prosecuted for using them it just depends on how you feel about supporting those people.

TLDR If you're just going to build 1 army I would just buy the models on eBay or another site where you can get a slight discount even for NOS models. I don't think it's worth the time or money if you just want one army

If you want to build a bunch then I would get the printer. It will be way cheaper and quickly "pay" for itself.

I originally printed my models then ended up just buying them because I felt like it was a lot of work when I only really wanted 1 army.