The KTM is expensive even second hand, especially if you're going through a dealer, and it'll only depreciate. Also the standard worries with a KTM, chavs love to pinch them so insurance will reflect that, somewhat questionable reliability compared to Japanese brands, even more so since the 125 duke is made in India.
My opinion of them is very similar to the CBR and YZF 125's, you're paying for the styling. Remove all the angular plastics and they're really not much different from any other 125.
As cyronn said, best suggestion would be to just use training centre bikes to get your test done. Learning gears won't take much. Failing that, if you want to practice and do tests without using a school, you might as well get a CG/ YBR, get tests done and when you upgrade you wont lose anything by selling it on, might even make a bit if you're smart about it.