Just before dd's 2nd birthday we did up her big girl room. We kept the nursery as is and she moved into the spare room. We purchased a double for the exact reasons you stated and when she is sick I sleep with her. It's easier to read her books before bed too. We bought a good quality mattress and frame so hopefully the bed will last until she is a young adult.
We are using a side rail from aldi. Best purchase ever. I feel much better knowing the rail is on. The other side is against the wall.
Day sleeps are easy, absolutely no issues there. Night times are harder. We still haven't taken down the cot about 2 months later and she does muck around most nights asks to go into the cot, then back in her big bed, back into the cot. It's frustrating but I think that's more to do with the cot still being up than her disliking the new bed iykwim?