elemental_funk, i spent $1500 when i first did the CA18DE+t converstion into my 1200 sedan and i had to piece together all the turbo stuff and flywheel and clutch from many different people so that made it cost more. i did already have the gearbox and radiator in there from the L series that was in before it. also had suspention and brakes sorted but i dont really count that as part of the conversion cost as everyone should be upgrading their brakes in a 1200, and the diff comes with power, you dont need to upgrade it but if you have a certain level of power you will probably start breaking them no matter what engine it is
in saying that though, i have since spent an extra say $3000+ to get it to the point where i want it, full forged rebuild, bigger turbo, better manifold, tune and so on.

I'd say CA is actually an easier conversion to do as far as mechanical stuff is concerned (in a 1200) given they are smaller. but in a 120y the larger size of the L series probably isnt an issue

i wouldnt go any of the bolt in crossmembers as i doubt any of them will really sit the engine in an awesome position, and making new mounts on the crossmember isnt really at all hard

a built turbo A series or L series would be pretty cool, a well made injection/turbo setup on an L series sitting in a 1200 (or 120y) engine bay would look quite nice in my opinion

and I'd agree with you that money, or skill and equiptment to offset the need to spend money, is a large part of what engine goes into most cars