I'm not sure I'd go for one currently, but not for the same reasons.
1) They will sell you essentially "a car in a box" i.e. all the bits you need to build one down to the last nut and bolt, but for about a grand or two less than a new factory built one that's IVA'd, registered, debugged to a great enough extent that it will at least drive, and on the road, as far as I can recall, so that's a mug's game, IMHO.
You can buy a modular kit where you get most of the bits required in a number of stages to spread the cost. You always used to be able to do a Q plated modular kit where you could supply the mechanical bits from a donor instead of new.
You can buy a starter kit which is basically just chassis, bodywork and the other Westfield specific bits, leaving you to make your own choices about all the other stuff.
2) The traditional SEI kit was based on no single donor. Sierra bits at the back to support an IRS setup and Cortina at the front because the McPherson strut upright on the Sierra is both pig ugly and unduly heavy when botched into a double wishbone arrangement. Power train is again a mixed bag due to no RWD Fords ever getting a decent enough engine(IMHO).
They do a Sierra "single donor vehicle" kit that contains said botch. Can't really see the point as you
won't want to be using the engine out of a Sierra (unless it's a cossie).
They also do a Mazda MX 5 based SDV kit which is much better based on the quick run around I've had in one. At least the Mazda running gear is reasonably light and you'll end up with a decent car at the end. Upgrade options from then on might be limited, although there are tuning options for the Mazda engines.
Unless you specifically want to base it on a single donor it's probably best not to, in many respects. Reasons for building a single donor kit would be to avoid a Q plate (without declaring it new) and .. well, that's about it really.
3) Mine took a year at quite a leisurely rate but it depends what route you take. "Car in a box" would take a couple of weeks whereas stripping a single donor, refurbishing parts to a decent standard and then building would clearly take
much longer.
4) Again, depends what route you take. There will be plenty, though.
With a kit car there is every option available from a labour of love starting with some lengths of square section tube and a welder, and hopefully ending with a car, to slapping down £20k+ and having it on the road in time for spring, and anything in-between. I guess mine was in the middle. I think the first thing you need to decide is where you want to be on that scale.
They can be very much better value 2nd hand, so have a think about how much you really want to build it yourself, as opposed to buy on 2nd hand and tweak / upgrade.