Hi Ian from Southern California and may I congratulate you on your great taste in motorcycles!
Just to add to Bill's solid advice, since your target C650 is a 2018 model, it will have the improved hydraulic cam chain tensioner (as opposed to mechanical, the switch occurred sometime in 2016) so that should not be an issue.
On the idle air control hoses issue, I have had to replace both on my 2016 C Sport and it seems like it's not a question of if, but rather when, they need to be replaced, so definitely inquire about that. The bike's ECU has the capacity to compensate for smaller leaks in these hoses up to a point, so it is possible that your candidate has this problem, but it is being 'camouflaged' by the ECU (until it's no longer able to compensate as the leaks inevitably get bigger). Replacement with new hoses is not a big deal and any shop, or avid DIY-er,with the workshop manual can do it in a couple of hours.
Then depending on mileage, things like drivebelt, transmission chain guide rails and oil, CVT rollers and sliders, need to be replaced at set intervals so look for evidence these maintenance items are up to date.
Let us know if you end up buying the scoot and we like seeing pictures...