5FDP has done an excellent job at thoroughly covering all your queries Chris, as someone who has recently just completed their own G1 Scorponok ; I can attest to the heavy undertaking that trying to piece one together can be, I'd also like to suggest that if you'd be interested in acquiring a near-complete/complete G1 Scorponok then look to eBay UK as there tends to be more listed in a better condition then that of eBay AU or US.
- If you want to go down the route of "Piece-Mealing" one together, then I'd say utilise all 3 eBays but if you want to try and go for a complete one; stick with eBay UK.
- He's going to be expensive to obtain and his value is on a steady increase as of late; my own owes me around $500+ via the piece-meal route but one could pick up an almost as complete example for $350-$425.
Also, do not - repeat - DO NOT try to move Lord Zarak's unless you're willing to risk breaking his entire arm off of the joint, it's a very common issue with Headmasters and it's best to just avoid playing with the arms at all.
Best of luck in your endeavours Chris, please don't hesitate to ask if you've any furhter questions as I personally love G1 Scorponok and I'd be happy to give you a hand like many others here.