I'd price it out. You may find it just as cheap to buy a new bike with Di2 then to get a used bike and pay full retail for Di2. The OEM gets much better pricing on components. I have bought and sold frames on Ebay with no issues (so far)I'm thinking about getting a recent, used bike from around 2013-14 and installing Di2. I'm looking at bikes that list for $3000-$3500, and I'm hoping to pay around $2k.
Where is the best place to look? eBay? Craig's List? RBR classifieds?