1. Should be relatively easy.
2. Cost depends on where you are. Probably a full day's work if you want the Megaflow removed, so around £600 in London, maybe £500 in another major city and perhaps £400 elsewhere. Plus possibly £100 to £150 for pipe and fittings.
3. The Megaflows are very well insulated. You might like to look up its daily heat loss in kW hours, and work out how much gas that means to come to a cost comparison.
4. Don't forget:
4a. You will have a much longer wait for hot water to arrive in the bathroom than you do with the Megaflow. Probably why it was installed in the first place.
4b. You almost certainly won't be able to run two showers at once, and turning on hot or cold may interfere with any shower.