You'll also want to get a good straight edge in a couple different lengths as well as some sanding blocks. I like the Great planes sanding bars. Their easy to grip and they're straight.

As for the razor saw, it's definitely a must-have. Make sure you get one with fine teeth. Small balsa wil be hard to cut if it's too coarse. X-acto sells a razor saw and mini mitre box combo that's very handy.

Fine tip markers come in handy for marking balsa. Soft balsa doesn't always take pencil very well and ball-point pins can crush it.

Another thing you'll need is a good flat building board that takes pins well. Some people like ceiling tiles, others like drywall. I personally like the boards that Great Planes sells. They are flat and not so hard that you can't get pins in them. Speaking of pins, make sure you have plenty of T-pins. small ones work best for going through balsa but the larger ones come in handy too.

Finally, I'd get a good assortment of small clamps. Harbor Freight tools has all sorts of inexpensive adjustable plastic clamps and spring clamps.