Oooh how exciting, its great being a granny bet you're so excited. The one thing I would say on cots is to watch out for Ikea. They seem to do funny sizes so you might struggle getting a mattress to fit an Ikea cot from anywhere other than Ikea. My daughters cot was bought for her off Ebay second-hand and I bought her a new mattress from Boots, which seems good quality. I then bought a second-hand cot-bed locally for my house as we have grandson to stay over once a week. My daughter seems to like our cot-bed more than her cot and I must admit now he's 1 years old, he does seem to have much more room in our cot.
Do go for a drop-side. Mine isn't, but luckily because my grandson is really taking his time at moving around we can still have the mattress set pretty high up. At first he was sleeping in a travel cot and it is back breaking leaning over a cot trying to get a baby to settle, especially one who thinks sleep is for the weak.