I had 15 mths between DS1 and DS2, and decided to put DS1 in a bed so the new baby could have the cot but this did not work for us as he loved the bed for the fact he could get out of it 100x a night. In the end we borrowed a cot for the new baby and had both cots in the same room until DS1 was over 2. However I know a lot of kids are happy in a bed at a younger age so it really depends on your oldest child and how they cope.

Do you have anyone who could lend you a cot as a temporary measure? I bought a new mattress following SIDS guidelines (although not sure if they are still current) but it was much cheaper than buying an entire cot.