From someone who is part way through an engine conversion and decided to build his own frame.
NONE!
They all have their issues. I'd say that the micra engine conversion is probably one of the easier ones to do but it is still a bit in its infancy so you will have less support from other people compared to if you did a vauxhall, rover, suzuki or honda swap.
I'd say that one of the cheapest is the 8v small block vauxhall engine as there are loads of these engines knocking around, gearboxes are easily interchangeable. They have fairly good tuning options afterwards as well so you can get a lot more out of them. Achieving say 150bhp out of a small block vauxhall engine is going to be easier than trying to obtain it out of a CG13DE micra engine.
If your goal is only around 100 bhp I'd be tempted to put the money into a 1275 turbo. I ultimately wanted more than this and 5 gears so I could use it more as a daily driver, hence I choose the vauxhall route.
After building my own frame I understand why Allspeed charge what they do for their frames. They have taken a lot of the guess work out of a conversion.
Its a case of balancing cost, time and skill. If you have the money and want it done quickly then you can compensate for a lack of skill, if you don't have the money then its going to take you a lot longer to do or you are going to have to be much more skilled in fabrication.
I'd advise that for any conversion you buy a complete donor car. It will save you money in the long run and makes life much easier to get everything from one source. I wish I had done this.