I got a 3-4-5 coverstitch/professional grade home-size Bernina ideal serger for a gift before the birth of my 18 month old, and I get free instruction in the Bernina shop, etc., and I've done nothing with it except seam finishing on a 4-thread for a long, long time. However. I would choose a good brand of machine which probably would be like any of the good brands of sewing machines: Bernina, Viking, Elna, NOT SINGER anymore, there are a few others, most aren't made in the US. My only personal experience is don't buy a Bernette--a small version allied with Bernina but not made by Bernina. They are not well-made. But Bernina is.
The idea of cards for tensions is great--with mine I really have to experiment with each thread type, and however many threads there are. I have never tried the coverstitch option. If I get going with this dollmaking thing, I'll have to see the power of the serger, b/c it will do things so well that I can't even do w/ std. machine. I'm working out a chart though, one setting at a time. I don't think mine is difficult to thread, and it came with a very informative but BORING video that isn't any good for just sitting and watching--you have to be trying out the thing.