Cylinder 6 in 5.4 f150 has low compression. Should i buy a new engine? Rebuild? If i buy new what should i get?

 

most of the time its cheaper to rebuild or repair. unless you get a killer deal on an engine from a scrap yard and even then it needs some rebuilding. just my 2 pennies.

 

Try a blow by test to see where the compression loss is.
A compression test alone does not tell you the whole story.

Unless you have all of the tools and a shop to do the work in, I would go with a reman long block. Rebuilding you have so many variables and do you have the time, talent and $$$ to over come the challenges that lie ahead.
And used parts are just that.