Brushless - thats to do with the engine, right?
Can I buy a controller that controls more than one boat (not at the same time obviously)
How aboutkits? Ant good?
2 families of electric motor - those that supply current to the rotating bit through brushes, and those that supply the current through devious magic to the fixed bit of the motor to make the moving part rotate., known as brushless. The brushless type are generally more efficient, and, lacking the losses introduced by brushes, can reach much higher RPM. For a given performance, brushless motors tend to cost less, but the brushless ESCs required make up for the saving. Terminology - electric ones are usually referred to as motors, engines are usually internal or external combustion. There has been confusion in the past.
Receivers dont really care about the transmitter and vice-versa, as long as they are in the same band and using compatible signalling, although it is usually safest to stick with one make, at least until the confidence has built up. The newish 2.4GHz sets seem to be heading down the path of every manufacturer trying to be incompatible with everybody else, and in some cases, not even with their own ranges. This might change in time, but as a guiding rule, one transmitter from a given manufacturer can be bound to, and work with, any number of compatible receivers, as you say, using one at a time.
Kits. Some great, some not, some need tweaking a bit. Best to lurk around the part of the forum dealing with the type that interests you and reading the queries and experiences, then ask again.