I would agree that if you are having to pay someone to do all the work, it is likely not worth it. However, it can be very beneficial and satisfying if you have the capabilities and tools(nothing fancy) to do the work yourself.
I bought a JD D105 with a blown 17.5 B&S in otherwise mint condition with low hours for $250. Pulled the engine which took less than 30 mins and 30 more mins of opening it up revealed it was toast. Purchased a brand new B&S 17.5 replacement for under $600 and installed it in 45 mins. Runs like a champ and it looks like new. So for a little over $800, I've got a great tractor. In my area of NE OH, used tractors of this size and condition rarely sell for less than $1200, which is about half of the new price for a similar unit. (I can reduce my cost even more by selling off parts from the bad engine like the starter, fuel pump, carb, head, etc. as the OP contemplated in their original post)
So in summary, I agree that if you have to write a check, used units of all sizes and conditions are out there. However, with a modicum of interest, ability, and a modest set of tools, it's a great learning opportunity. Enjoy!
ps: I'm sure there a models out there that are a PITA to disassemble so your mileage may vary! I have a fair amount of experience working on these which is why my times are fairly quick and I thoroughly enjoy the work.