Merino wraps are awesome.
We have muslin wraps and merino wraps. We rarely use the muslin wraps because the merino ones are great at keeping him warm in colder weather but not making him too hot in warmer weather.
I would say get both short-sleeve and long sleeve tops and aio's. Sometimes Jude gets hot during the day so we sleep him in just his singlet but during the night we always sleep him in long sleeves (he is sleeping with his arms out of the wrap now, so his arms get cold).
It really depends on your baby and where you live though. We live in Dunedin and Jude is not a very hot baby (he usually likes a blanket on him for sleeping even in hot weather - I'm the same

). However, my sister lives in the Bay of Plenty and uses her muslin wraps a lot! I think her baby must prefer less solid blankets on him than Jude. When we were in the BOP over summer - Jude still preferred having the merino over him rather than just a muslin when he was sleeping.