I bought an 04 Fozzy XS, no turbo, with 114 K on it. Part of the deal was to have the dealer get the TB done at his cost. I took it in and had it done, but as I drove for the next 50 miles, it kept getting hot, I'd stop, wait, etc. did not abuse it. Took it back and they said it was broken end caps on the radiator. They denied breaking them on the TB job. OK, had that replaced, left the shop and within a mile it began losing power and ran rough. The shop claims they ran it for over 4 hours testing it and it was good. (Sure they did!)
Shut it down and had it towed to another shop. Head gasket blown. Tore it down and that shop said the oil was milky and the warranty company sent another engine with 61K miles on it. Then while there was no core charrge with the warranty company, the second shop decided they wanted my old engine as a core charge...LOL, not, it's my engine! The wanted 200 bucks for it....So, Monday, we will talk again. My position is that if the parts provider has no core charge requirement for any part, the old parts are still owned by the owner (or the owner can pay the core charge), not up to the shop to assess a core charge they are not required to pay. And, no notice of such policy was made. I'm pretty sure I will get my engine back! Your Honor...etc.
I believe the water got with the oil while it was not running, as it was not driven or started, so with a good flush, it should be good and have the heads milled, it should be good to go.
So my advice, if you are not a wrencher, contact some shops in your area and see what they think of Subbies, some here won't work on them....maybe because they don't know them or they are not profitable, don't know why.
Find a good shop that you can work with, have them look at it before you buy.
If a rebuilt engine drives out OK and it has a compression check, no noises, it should be good to go! IMO And all the better for new miles on the engine, follow the break-in proceedures. Good luck....LOL