When you do the junkyard motor you do the gaskets and seals while the motor is out so you won't have to worry about it later. Head gaskets should be fine so you don't have to do those. Lim, oil pan, timing cover, rear cover, are all gaskets that will eventually start to seep, it's less work to do it while he motor is out. Felpro makes a gasket kit that's only $200ish. Could do a cam or rockers too while out, again just easier while it's out. There's a nice how to pull a motor billboost style in the how to section to help you out also.

Rebuilds typically don't last on these blocks so you'll be much better off with a junkyard motor.