Very good point, Paul, I'll try to give more of a detailed description of the scenario:
The problem we have is that we are in an area of relatively high rainfall as we are near Dartmoor in Devon.
The soil is clay - we have about 2-3" of pretty decent clay-ish topsoil, then gradually becoming more clay. About a foot under the surface, we then hit solid grey clay. I've dug down about 3 feet and it's still the thick grey clay. How far down, I don't know yet.
We don't have much in the way of hills around us that run water down onto our land, or rivers, streams etc, it's just that the water that is there from that rainfall, just sort of sits there on the surface. It's a bit soggy in summer (soggier this year due to the rain we've had) and in winter, there is standing water on the surface. It's not underwater as such, but not far off.
We are planting up as many deep-rooted plants as possible, to try to break up that clay soil a bit.
Beyond that, my thinking was to create some off-contour swales to run some of that water down towards a pond, which will also provide a more diverse landscape, with mounds, swales, small ponds, hugelkultur beds, therefore lots more edge, and some different niches and microclimates to play with.
Willow is in my plans to soak up some of the water, as is bamboo.
This will all be done by hand, so we were going to put in one swale, and see how much of a difference all this makes to the area above the swale, with a view to putting in more if all goes well with the first one, only if we're putting in more, we'll hire a minidigger.
So, the quick answers to your questions would be:
3. Yes, although there is some slope so some areas are wetter than others
4. Yes, that's certainly a consideration.
One further problem (although also a lovely benefit in some ways) is that we are in an area of culm grassland, a local wetland habitat which is a depleting habitat for endangered butterflies and other creatures. We have consulted with our local Wildlife Trust, and on their advice, are going to leave around 1/4 - 1/3rd of the site (the wettest part that is slightly downslope and out of the way from the area we are looking to drain) as grassland for habitat.
Where the grassland has not been managed, though, we have very tall soft rush, which has totally dominated. Where we have started cutting this to encourage a more diverse mix of grasses, the ground is getting slightly less damp as the sun and wind get to the ground. We're doing all of this slowly and all with hand tools to begin with, in order to observe the changes and learn as much as possible about the land, rather than rushing in with the diggers and making possibly mistakes that can't be rectified.