totally agree with the gro clock, def recommend that as worked a treat here. (cant wait for ds2 to be old enuf to use one!!!) ds1 went in a BBB at 3yo, we delayed as no way was i doing any return to bed stuff while doing early morning bfs when ds2 arrived. (tho he's never actually gotten out anyway, i'm assuming because he is older and well aware of the 'rules')
i also think he stays in bed because he is still in his grobag so restricts walking too much. didnt stay under covers enuf when we moved him in but he's pretty good at it now so will ditch grobags in summer. so i'd recommend that too if your LO still sleeps with one on.
we went for single bed even tho his cot can be a toddler bed as he was older and we got him a car bed so no need to worry about rails, falling out, etc. and it is very low to ground also.