Unless you have one heck of a garage (and know how to do it), the machine shop will be doing 90% of the work on the block.
Hot tank, crack checking, line honing, bore honing, resurface the decks, balancing, ect.
Not to mention ring gapping,
oil clearance adjustments, end play...
If the machine shop screws up a "crate" short block, they'll screw up your stuff.
Once you start pricing stuff, you'll find assembly (which you may not do right) is a fraction of the engine's cost. Sorry, but asking the difference between a Merc and a Ford motor tells me you may be better off having a pro handle it. There's lots of stuff to be done wrong or missed that'll cost you down the road.