I think you've made a good decision...well thats what we did and it worked for us and DS1 was 20months old when DS2 arrived.
We bought DS1 a single childrens bed when he was 18months old- started with day sleeps then moved onto night sleeps about 3weeks later. We've not had too much trouble with DS1 getting out of his bed either and he took to sleeping in it pretty well. I think it helped that when we went looking for beds for him- we watched how he'd react when we went to the stores- we ended up with a train themed bed- because we looked at it a few times and each time DS1 would run up to it, climb on and lay down and pretend to sleep.
We also put side rails up, which kinda enclosed it (except for the bottom corner- so he could get out)- which i found had a little bit of a cot feel to it.
At least also with a single bed you'll still able to lay in there with him- though it might get a little tricky towards the end.
My back up plan was to use the port-a-cot too- got it all ready but didn't use it once. So hopefully everything works out for you's as it did us.
