Already have a nowtv account and a white box on one tv, working fine for years, so no problem with that.
Used to also watch on an LG tv that already had now tv app with it, but LG tv not working anymore, so will be buying a sony smart tv as replacement, but what's the best way to add now tv to it without the least hassle and technical problems? Another box, I guess? Which is fine, but will there be any problems linking it to my existing nowtv account?
I believe at the moment Sony Smart TV's don't have the NowTV inbuilt App, therefore your best option would be either one of the nowtv boxes or perhaps the new nowtv smart stick coming out this month.
With your existing account you can have a maximum of 4 devices with 2 simultaneous streams, if you have reached your maxium device limit you can swap a device slot by going to My Account > Manage Devices.