PRAM batteries are pretty cheap on Amazon.

There was an option for slot loading DVD drives. Find one, and you could switch it out. Video files are pretty much hopeless on these now. They're good DVD players if you can get a DVD drive in, but don't count on much for video files of any sort.

If Diablo asked for Classic, that means it was written to run on an older Mac OS (think 8 or 9). You should be able to run it in Classic mode if you installed that with 10.3.9 (I'm kinda fuzzy on details, maybe someone else can chime in here. I'm not sure if you had to have classic installed first or if it came with OSX).

If your wifi USB sticks are compatible with 10.3.9, I don't see a reason it wouldn't work. The USB is 1.1 though, so don't expect blazing speeds at all.

I have loads of extra speakers for iMac G3s... if you'll cover shipping, I'll send you a few. (Edit: I see the prices you mentioned above in Euros. I'm in America, so shipping might be prohibitive. Though the offer still stands if you can't source anything from that side of the pond). The tough part is getting into and out of the case without damaging it. Since the G3s didn't have an internal fan, they ran pretty hot. That, combined with time, leaves much of the internal plastic brittle and very difficult to work on without snapping pieces. Might be a better idea to get external speakers instead of risking replacement if your unit's in good shape now.

If you do open it up to do any work, be careful with the CRT. Google around to find out why if you're not familiar.

https://www.amazon.com/Xeno-Energy-XL-050F-Lithium-Battery/dp/B005NI59W8

Some models of old Mac actually REQUIRE the battery to boot successfully. It could be that without the battery, this one needs to build up juice in the caps before it will boot. (