My girlfriend and I hiked Volcan Baru in July. We debated on whether or not to get a guide and whether to stay overnight or start the hike at midnight.
Now, hiking it is something I loved doing and is well worth it, but you don't need a guide and if you can't camp outside don't take the offer for "shelter" at the top of the mountain. The path is very clearly marked, you're not going to get lost. And while I personally liked our guide, it was a waste of money. The shelter we were provided was sleeping on some ridiculously dirty mattresses next to the servers in one of the buildings where all the wireless towers are. If we could have hiked and camped on our own that would have been better but camping was not recommended at that time of year.
Don't get me wrong. it is beautiful up there. The hike is hard but if you are in decent enough shape and take your time you will make it. I have no regrets about the hike, it was just that we spent money we didn't need to.