Sorry but your 1977 JD 300 shouldn't be put in the same sentence with a newer 100 series with K46 transaxle (L, X300, or X350). The old Sundstrand transaxle and horizontal shaft drive has a design life WAY beyond 350 hrs.
To the OP's question of engine choice... it doesn't matter. Any engine you pick will wear out three K46 transaxles before dying.
I'm a retired JD L&G engineer. Inside Deere MANY of us have the same machine: 2000-2005ish 325, 345, GX325, GX335, or GX345. Why? Because they are the pinnacle mowing tractor for 1/2-2 acre yards. There are other great tractors made before as well but the decks?... not so much. All have K71 transaxles (the GX tractors have diff-lock), really good mower decks, are smaller statured, and still have all the premium do-dads that make mower (and attachments) easy to remove. Most guys have the GX345 (KHI liquid cooled) variant because there are more of them out there. I prefer the GX335 because the Kawasaki big block air cooled engine is bullet proof... there is just rarely any warranty issues with them (that's why they're still in the JD lineup and the liquid cooled is not!)
The new JD X5's w/ K72 are great tractors. The new decks are AWESOME.... just $$$ My GX/48C was $2500 3 years ago with 200 hrs. I'd look there. Hope this helps!