Megaflow or not?

There is no right or wrong answer to your question it is a matter of preference. I would personally choose the unvented hot water (megaflo). On the assumption that you plan on using the system at its full load (using all outlets at once). If you use 3 pumps that are of a good quality and power for longer enough then not only do you run the risk losing hot water but theres a possibility that you will drain the system of water and the pumps will run dry. Which if done enough times could end up being quite costly. Also unvented cylinders have a faster recovery rate than your average open vented cylinder. The volume of water stored in the unvented cylinder is larger than the vented cylinder of the same external dimensions. They are also more energy efficient. The noise of three pumps running at once could become quite annoying some times even one. If the condition of the pipes that you currently have are not good then the system suggested by your plumber is probably the cheaper option. Lastly the standing pressure is a good indication if you are able to have a unvented system installed but you will also need to check the working pressure and flow rate. Do you have 21mm or above main cold water supply?