AlanE, you definitely can reuse many components, especially if you plan ahead. You mentioned wood saw. You can reuse engine, and whole hydraulic station. It is good idea to add some sort of oil cooler in return line because continuous use of hydraulic station heats up oil.
Drill pipe.
Do not go smaller then 1 1/2" because if you hit rock and need to use trycone bit you will need to make sure you can push it down. My pipe looked very solid even when whole rig was "dancing" in the air. If you go with 2" you will find pipe and couplings are very expensive (I used 1 1/2" hydraulic couplings grade 3000psi rated they are about 20$ a piece). You can find a good use of pipe if you think. You can use them as posts for fence or ...
Try to find king swivel. It is the best for projects like yours. It has special type of seals to handle sand and mud. You can see many other ones use just regular type of seals or graphite rope.
When it comes to drilling you will find the paradox. The more you read and know the more questions you have. The main problem is you are drilling an earth and you have no idea where you are drilling and what is there. You asked me about water table. My answer: I don't know. When you use mud your hole is full of it and even if you hit water table how would you know? If you use a shovel this is easy: "here is water coming
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