I would recommend a smaller diameter drum (14" is ideal, but failing that 16" is still better than 18"). I use a 14" bodhran made by Albert Alphonso (www.celticmusic.com)with a small footstool to raise my left leg and to help keep me upright. (I have a tendency to lean forward when I play and this keeps me from overdoing it.) Iwould strongly recommend lessons with a live person as opposed to (or in addition to) a tape. It's easy to think you are following a tape correctly and an instructor can give you feedback based on your playing and help correct any mistakes you might be making. You don't want to end up practicing a mistake until it feels right. It's hard to learn but it can be even harder to unlearn!
As far as a choice of drums I absolutely love Albert's bodhran's but he is expensive, and you wouldn't want to spend that much money on a drum just to see if you really want to play it or not. Seamus O'Kane in Derry makes a good drum and is supposed to be reasonably priced. Brendan White (from Holland) makes a nice drum but I don't know what his prices are like. Look for a thicker skin if you have a choice and definitely go for a tunable drum if you can. It's worth the extra expense. A couple of don'ts: Don't buy a bodhran wiyh a beer label (or any painting, for that matter) on it. Guinness drums are nice things for a tourist to hang on his wall but that's about all they're good for. And PLEASE don't borrow a drum and play it at a session before you learn to play it. I wouldn't go to a session and borrow somebody's box or pipes to see if I wanted to learn.
Best of luck,
Rich

Feel free ro contact me if you want: rrayburg@hotmail.com