1) The names are very similar to the Rectifier series, with 50, 100, and 150 watt amplifiers. Ace = 50, Deuce = 100, and Trident=150.
2) No, the Ace was introduced at the same time as the Series II Deuce and Trident.
3) I'm not sure, but the Manfield kit that was popular on here is no longer available. I'm sure that a qualified technician could do the mod for you, though.
4) Search this forum. There are a lot of mods that people have done, and I don't know the details of all of them. The simplest, obviously, is trying different speakers.
Personally, I have an Ace combo that I love a lot, and it's stock. I will definitely try different speakers at some point, but for now, it's good enough. For reference, I'm not a high gain player. I don't shred and I don't like metal. I just like good tone, and the Ace gives me just that - good tone. If you get the chance to demo one, you'll be very pleasantly surprised by how great the clean channel is, especially for running EL34's in the output section!