I had 80 gals of fuel and carried an extra 30, you will only need this extra fuel until the Marquesas as from this point good fuel is available at a price. Water is an issue so fit a good water maker as the only good water is available from Tahiti and major centres west. DO NOT TAKE ON WATER IN THE MARQUESAS.
So how much fuel did you use getting to the Marquesas Simon? Your tanks hold much more fuel than the Morgan NA-40 (which shows less than 30 gallon capacity); so how much fuel tankage or jugs should he bring?
I tended to rely on the headsail and kept a third reef in the main for most of the trip.
That makes sense... Only make the main larger if you need it and use the headsail for most of your power which you can easily reef or furl if heavy weather approaches.
You will need to check and make sure that the furler can handle reefing; find out what the model is and contact pro-furl or ask for documentation from the owner. In terms of dropping a headsail on a roller furling system the headsail must be fully un-furled then you just release the halyard and drop the sail down onto the deck. The sail engages a track (called a foil) and when you drop it down the luff tape comes out of the groove and is free of the headstay. This is easier to do in light winds; but can be more difficult to rig if you are in heavy wind and seas than attaching a hank-on sail. You can't easily drop/hoist a sail that has a luff tape unless you are head to wind; so that can be an issue if you need to change sails in heavy seas. For these reasons I suggest that your cruising headsail be heavily constructed to minimize the need to change it and make sure the furler is capable of reefing so you can adjust the sail area (or furl completely) easily.
Some other things to consider are the age of the standing (and running) rigging; which could be costly to replace if it was not refit recently. I would avoid rod rigging unless it is relatively new and there are receipts to prove it. You don't want to lose a shroud, halyard, or the mast while you are on passage.
To go via Marquesas/French Polynesia you will need to provide a deposit for emergency travel out should your boat sink. How much was that per-person Simon? It's refundable; but you need to be sure you have enough cash (or credit?) available to provide to the government when you check in.
So to make the boat ready you might be looking at:
Solar Panels and Charge Controller
Wind Generator or Tow Generator
Watermaker and spare filters
Standing & Running Rigging, Chainplates, Lifelines (negotiated into purchase price)
Spare engine parts (belts, hoses, impeller, zincs, heat exchanger, filters, gaskets)
A good survey of the boat before you buy; and several other boats to look at before you come to USA to buy.