Sven, looks like you're making good progress.
Radio:
The 6YG is a non-computer model. It gets you 2 more channels than the 4YF, but not much more. If your budget doesn't work for a computer radio, just buy a cheap 4 channel. The new Tower radio is probably from Futaba, much of their radio equipment. I know nothing about it. Something to keep in mind is it's hard to tell when an item that shows a stock status of "Order pending" will actually be available. You could end up waiting months.

Servos:
It looks like the plane is setup for standard servos. It's certainly large enough that their weight shouldn't be an issue. You could switch to smaller servos, but they are usually a bit more expensive. You would also have the hassle of adding something to the servo tray to accomodate their smaller size. However, if you plan to stick with electrics, and your next plane is smaller, the small servos might be a better choice. The S3101 servos would probably be ok. Alternatively, you could explore one of the Hitec 6 channels and equip it with small servos and the Electron 6 receiver.

Batteries:
LiPo batteries require extra caution when charging, and you also need to be sure that the speed control low voltage cutoff can handle them. If you or your son are not prepared to handle their special requirements, you may want to wait till later to go that route. A compromise that would give you less weight, or more capacity, than NiCads is to go with NiMH batteries. They weigh less than NiCads, but don't have the special care issues of LiPo batteries.

Chargers:
Yes, they can be expensive. I've got an Accucycle Elite. I think it's great, but the $150 price tag is steep. The Quick Field Charger MKII doesn't cost as much and gets good reviews: http://www2.towerhobbies.com/cgi-bin...?&I=LXCTZ5&P=0