I should say that my knowledge of n gauge is limited but I assume the same principles apply as for OO.
From the picture, it looks like the only place you are looking to isolate is the ends of the sidings.
The exact placement of the isolators depends on the use, if you are just ferrying short freights about then it may be more useful to have the isolators nearer the entrance to the siding, or have multiple isolators, as, in real life, drivers will only take trains as far as necessary and, to be honest, it could be tedious running them all that way if you don't need to. Also, if you are running the trains to the end, the isolator could be placed slightly nearer the entrance as it would require less exact parking.
The positions of the power supplys seem okay for the layout.
It seems as though you have a set of points where you want to put the station, is there a reason you think they should be there? Generally there are only points at a station if there needs to be. Perhaps some of your trains stop there and head back, but if not, do you need them?
You should also consider how many trains you intend to use, because they have to go somewhere at the end of the day, and if it is fiddly to get them out it will only end in you using them less. Also there doesn't appear to be a way to stable trains in the anti-clockwise direction.
Also, if you want you layout to look like a real railway as opposed to a model railway, great care should be paid to the whereabouts of scenery. Always remember that the railway goes through the scenery not the other way round. Equally, the railway serves the community, so the station will be as close to the town/village it serves as possible, although some stations have moved.
A good example of this would be Arlesey on the East Coast Main Line (ECML). The original station was quite close to the centre of the village, but it was closed and subsequently demolished. Later, a new station was built but because new residential buildings had been built on the old site, the station was rebuilt at the end of the village.