Apple won't let you download Yosemite anyway, they always make you do the latest. Even if you're on 10.6.8, it's straight to El Capitan. They know best, huh.
How much RAM in it. Without an SSD El Capitan slows up older machines.
Unless 10.9.5 isn't doing something you need stay with it. I'd look into replacing the HD with an SSD before going any farther. An SSD breathes new life into old machines. It speeds up those Core 2 Duo machines by a factor of 3 or so. It's a lot cheaper than a new machine.