I used a wetsuit when I went through cave diving training. I was comfortable in a 7mm with a vest and hood and I tend to get cold pretty easily. My reasoning: I’d owned the suit for a while and was familiar with it. I didn’t want to have to deal with a being a dry suit newbie while worrying about my lines, buoyancy, etc. Also, cave gear is hella-expensive and I wasn’t ready to invest in a drysuit since I live in Florida and would have no other use for it.
To sum up, you won’t have any issues putting off purchasing a dry suit for a while. Also, you want to make sure you leave yourself plenty of time to get happy with your drysuit (or any other gear) before taking it in a cave.