You can certainly get around Arran on the roads, I crossed from ferryboat to ferry port on tour a few years back, on a recumbent. There's really one big hill in my experience, it was tough but doable with light credit-card touring load.
I'm trying to remember geography, I seem to remember it being fairly flat between the distillery and the ferry port arriving from Ardrossan. Mull of Kintyre has challenging slopes to get onto the top, 14% ish, bottom gear territory for me 30 x 32 I think on that occaision