Questions about replacing my high mile engine.

6500 is what I was quoted for a crate jasper 5.3. I have a feeling that would be the best route to go. I have found some used 5.3 engines with around 100,000 miles for around 2300, plus about 1000 for install. That is quite a bit cheaper than doing a crate , and would leave me extra money for a trans some day? Just a tough call which route to go? My motor is still performing quite well, but oil pressure is low at idle.