Heya!
Basically, go for it! I went to Borneo with RI back in 2004 and had an amazing time :-) I spent a month in an area of virgin rainforest doing an environmental project, then a month building a school in a village and then a month trekking through the jungle and learning how to scuba dive on a remote island. I had an awesome time, though it was tough at times, and now i'm looking to go back as a project leader in a few years time.
It's not a middleman company at all, the projects you work on are all funded partly by RI, so you and your group of 10 or so will be the only ones volunteering in any place. You have two project managers who are with you and help you work as a group to get your projects finished. It is a bit expensive, but that's cos it's an expedition company which actually trains you in how to live in the jungle, and then provides you with everything you need for three months, rather than asking you for much less money and just sticking you in some monkey orphanage somewhere :-)
Of course there are down sides, you're bound to get home sick, the work can get pretty tough, and there will be people that you won't get on with. But the friends you make and the fun you have more than make up for all of that. Waking up in the rainforest in my hammock, listening to all the birds and animals is one of my favourite memories of all the travelling I've done, and that's quite a lot.
DOn't worry about meeting people to travel on with, you'll find that a lot of the group have the same ideas and it'll be easy to find people to travel with. There are 1 or 2 training weekeneds before you leave for Costa Rica (I assume), and you can start finding out people's ideas then. I was getting offers of travel partners from the first day of expedition right up until the morning after expedition finished!
RI's a friendly organisation, and nobody will mind if you phone HQ o ask some questions, or join one of the many facebook pages to ask for advice. If there's anything else you want to ask me, feel free!
Oh, also check out on the RI website if there's a support group near you, they can sometimes help out with fundraising. I also got loads of money by spending an afternoon in the library, writing letters to trusts (look through a book called the directory of grant making trusts, or summat like that. It's full of trusts which will give you money if you ask nicely :-)
Cez x