From your description, I would suspect total re-plumb should be on your platter somewhere, probably with some priority. Blowing out the lines will gain you next to nothing. Maybe temporary reprieve, but not much more.

Household water supply is basic....as in BASIC! (notice what a gentleman I am....did not print that in bold/italic/36-point...which I was tempted to do.) Galvanized lines of this age are nuisances today and time-bombs for tomorrow. I suggest putting re-plumb high on your list of things to do.

PEX can be useful in remodel applications but only your local building code folks can tell you if it is approved for use in your area. I would also suggest making sure your plumber has worked with it before.

PEX and type L untempered (in coils) are going to be your friend.

There is no reason to even remove the old galvanized any more than necessary, just run new lines in the easiest path.

Just like fishing electric wiring, supply lines can be fished into closed spaces (you just have to plan access to make joints).

If you have more than one story, plan on using a single stud bay for electric and plumbing on the lower floor.
This minimizes wall damage and repair costs.

Blowing out old galvanized is not going to do anything.
The pipes are well past their useful life and likely almost (if not actually) clogged with hard scale almost completely.

thanks for all the info. I'm a bit frustrated as we had asked for a quote from the plumber during the inspection phase of the purchase process and he did not mention replacing the pipes at that point. He had included the 'blow out' only in his quote (which we used in making a decision about a price reduction to the sellers).

It looks like we'll be upgrading the piping within the next several months even though it wasn't in our original budgeting plans.

I will be getting another quote and asking questions re: PEX. There's decent access from the crawl space and also to the upstairs bathroom via a closet which was already opened up inside for plumbing access...

"I'm a bit frustrated as we had asked for a quote from the plumber during the inspection phase of the purchase process and he did not mention replacing the pipes at that point."

Incompetent would be one way of describing anyone who thinks 1927 galvanized pipes are still viable.

Just about any galvanized lines still in use are well past their life.
They stopped being used around the 1950s in most places, and that makes them at least 60 years old (let alone 1927 pipes).

Even galvanized drain lines that old are pretty much shot.

Cast iron DWV often lasts much longer than originally thought, but still needs a careful inspection for leaks and cracks.

Buried cast iron is on its last legs.

I replaced the galvanized pipe in my 1952 house in the 1990's. If you tried to look through a 10 foot section of the removed pipe, you couldn't see light at the other end. Think of clogged arteries, only the clogging is caused by nodules of rust and mineral deposits...which is not going to be dislodged by the voodoo notion of "blowing them out." Take the advice of others here and get the pipes replaced as your first upgrade to the house.