OK, finding land: it varies but is easiest if you are hoping to build in the area in which you alredy live. Often it's a case of pounding the streets, looking for likely plots and then researching ownership at the Land Reg or approaching owners with a large garden to see if they will sell part (although new legislation means that it may be harder to build on gardens from now on). Some land comes up in auctions and through estate agents but the best sites rarely come on the market, it's a question of persistence and seeing potential. In areas such as London and the south east, the only way to do it realistically is to buy an existing house and either sub-divide the plot or knock it down and rebuild something you lie better.
Finance - well it depends, some people have savings etc, some have enough equity in their current home but there are also mortgages from specialist lenders (there was an internet based provider called Buildstore and the Ecology BS also loaned for self-build but I haven't really looked into this since the financial crisis, I don't know if they still do). If you need one of these mortgages, the advantage is that they lend in stages so if you need, say, #300k for the build, they will lend you #50k to begin with and then when you reach an agreed stage, another chunk. That way you aren't payin interest on the whole amount from day 1.
How long the build takes depends on the size of the house, the design, the site, the method (timber frame, block work, SIPS etc) - impossible to say but almost always at least double what they thought would be the absolute maximum.
Whether you live on site or not depends on your finances and how closely involved you want to be in the project. It seems to work out for retired couples who are viewing it as a life project together but everyone else grows pretty fed up of it pretty quickly (especially as it always takes longer - see above).
Most people who are approaching a self-build whom I meet either use a kit where the costs are fairly specific and only go badly wrong if they change things around a lot or use tender process which again means they have some control on costs. Where it goes badly wrong is where they either start off very hopeful ("I've got #250k, so that's what it will have to cost") or they think "in for a penny, in for a pound" and just keep upgrading the spec the whole time. The most important thing I can see when self-building is to nail down every last detail - every last detail - and get the spec completely watertight. That means thinking about every socket, where it will go, what it will look like, who the supplier will be etc etc.
Most people I have spoken to would do it agan but most people I see have had very successful builds and end up with something that it would be very difficult to buy. I'd say very few of them realistically make a profit (they either madly exaggerate the final worth of their house or they haven't paid the full cost of the plot etc so if you factored that in they would have broken even or perhaps made a loss).
HTH