1. A$2000 is okay for the condition(quite a bargain if it suits your need). Of course provided it is as presented/conveyed - no surprises. Do the piston seals seal properly? They look a bit odd to me but I've no experience with that speciic model.
2. Yes you can, with Flojet running off a 5 gallon bottle. But plumb in is recommended for this type of machine. You will want to keep this on 24/7 - the boiler is too much to run on-demand.
3. If it's not designed to run off gas originally, probably not without some heavy modding. Regulation and safety is another issue if you decide to run on gas.
4. How are you bringing it back? Will there be additional import tax? Spare parts available? You really will need to plumb this in and run 24/7 if possible. 2 group for home is honestly a bit overkill (IMO of course).