Wsup, and welcome to the Supra life! Tricky thing with buying any used engines are that you never know what condition they are in externally and internally. Especially with used low mileage engines. But I`ve had 2 friends buy 1Jz engines from ebay and they`ve had a good engine given to them. I myself have bought a 7mgte low mileage engine from the website
http://www.jdmenginezone.com/
I took the risk cause it was just about my best option, and I got a pretty solid engine, rust was minimal and all I`ve done was change all the seals, gaskets, and basic maintenance. And I did the swap myself over the summer so no labor costs on that, just time I had to fork up, but it was really fun! Keep in mind this was back in 2012 and its still running strong with all my mods till present day, putting on about 25k miles total so far. So its kind of hit or miss with these kind of purchases. But for rebuilds it really depends on the shops around you, cause of parts + labor can really add up. So that I would not be able to give you insight on. Hope this helps