Definately go for an older cat. You can tell the rescue exactly what your existing cat is like, they can find one of a matching nature and they'll be on hand to help with any advice and questions during the early stages.
When I lost mine, I knew she was heading downhill for a while as she didn't respond to CRF treatment. Josh had already started to act weird and that only got worse when she left us. I wanted to introduce a new cat straight away but oh said "Is that for Josh's grief or yours?". He also said not to go looking and that the situation would resolve itself.
I got involved in rescue as a result and almost a year later when Corporal Smokey came in I knew enough about the cats in our care to know that he was the right one. Josh was still howling at night and hadn't settled. Even though I'm a volunteer at the branch, the ladies said they were always a phonecall away and if it didn't work out I could bring him straight back.
That was really nice to know and I'm so glad I went for a cat that had a developed personality.
In hindsight, although they get on, I think that Smokey is a little kittenish for Josh sometimes (they're 2 and 13) so, heaven forbid, if I were in the same situation again, I think I'd go older still, maybe four and over.
Whatever you decide, I hope it works out