I made a mistake in my calculations, but noticed it too late. I tried to make some clarifications, but... I'll fix or delete it later. I hope someone writes a better answer and makes this one obsolete. It still has some info for the OP (otherwise I wouldn't post it) so... until a better answer posted.
Spinning Ceres isn't such great idea, as @notovny pointed in his comment. It's also easy to see by such an example - let's assume that your people are 10km below the ice, shielded well, with their feet pressed to the ceiling with 1g towards equator. But all the ice, which is under their feet, wants to fly to the equator. It can be visualized (not an exact match) as an ice sky scraper standing 10km height in the earth's gravity. Even if both cases are not equal, the ice will get what it wants: To fly out in one case and crumble/fall down in another.
This is not a huge problem, just makes your happy version of an ice express. A torus and maglev will fix the problem.
- In that sense, why not to begin with building that torus next to Ceres, using Ceres' materials? Okay, there are cons and pros, and in my opinion the pros of carving out an asteroid doesn't outweigh the cons, but at least it has a couple positive places.
As for the energy required, our Sun has more than enough energy for that, so in theory it shouldn't be a problem. If efficiency is high, it will probably be capable of accelerating at 10 m/sec2 (1g) or something like that, minus a mile (to start).
- If they have access to even one percent of the Sun's solar energy output, then it can be done quite easy energywise, but such a choice doesn't have much justification. Ceres travelling in this way is only a result of some story, as without some reason, it doesn't look like a good choice. But sure, the story can fix that (It's a bit of a lazy move, but people eat it).
Using electric propulsion and current engines specs, you will use about 36% of mass of Ceres to reach 0.1c (with an exhaust velocity of 0.1c, which is the fusion engine's speed).
WARNING I made a mistake here with exhaust velocity, which makes up the rest of the answer. More importantly, the conclusion is invalid. In the case of ION engine, things are grim, but if it upgraded to the level of fusion engine then things look a bit better, except for the energy consumption and the time required.
Wrong numbers, Wrong conclusions
The correct end mass for Ceres is 30km/s exhaust velocity. There is no way to have a 0.1c end velocity, as it is 21x of the exhaust velocity, and then you will be left with about a million tons (10^9 kg) of ex-Ceres. (The rest is spewed as reactive mass.) So really you barely get to 600km/s, and starting from Ceres orbit, it's way more than one needs to escape. (600km/s escape velocity is next to the solar surface, and at Ceres, the orbit's main job is done. It is like the energy was never spend/lost.)
In engines, their TWR is an important number. It produces 5N, but how much mass it has is on its own. It is about 230kg, so max acceleration is 1m/s in about 46 seconds. This means the best time it can possibly be in this case is about 43 years to reach the speed of 0.1c. In reality, it may be a couple of times longer, depending on how heavier Ceres is than all those engines. For every 10 times larger, the trip is about 400 years later. That is probably a reasonable number.
- I have reasons to believe that scaling it up will improve specs in the TWR aspect, but you would need energy sources, places to store the energy capturing devices, etc, so the estimate above may be a reasonable number in scaled-up version as well.
All in all, things aren't that bad with fusion, (like ION engines). It may need a few tweaks here and there, but it can become a reasonable generation ship. However, with the ION engines, which have 1000 times worse ISP(exhaust velocity), it is quite bad. Nevertheless, energy source is biggest issue here in both cases.
So one needs about 470 trillion(4.7x10^14) of such engines, and power consumption/generation 47'000'000'000 GW.
Post-error detected explanation: As we will be left with nothing, for a mass as large as that of Ceres, especially with those low ION engine ISP's, we have to increase ISP. We'll have the same improvement basket, but it also means that if ISP is increased 1000 times to the level of fusion engine, energy consumption also will be increased, but at a million times increase. (Energy is the square of velocity.) Sooooo... That means that it is 1'000'000x larger than of those 47 billion GW. Sooooo... Ring diameter will have to be 1000 times bigger. OR it has to be a million times slower. OR a combination of those two.
47 billion GW is an energy-capture sail/system of 200'000 km in diameter at earth orbit (1300W/m2). Not impossible as well.
You can, however, use oxygen as reactive mass as well. You'd have to repair those engines and replace them along the way, which means factories to make all that, all the technology for all that, and many, many more other things. Oxygen can be ionized in the same way as hydrogen, keeping the high ISP. Take it from the improvement bucket, where all that scaling up lives, and it is quite a minor upgrade (and there are ion engines which work on air/oxygen containing mixture.)