value for money wise id build a 1.8 turbo lump with kms management cusco diff far cheaper easier than getting a cossie lump in there. As far as im aware flyin miata us still make the best turbo kits i have had a manifold off them for mine and the quality was great

as for the sr20 the good ones are few and far between (and getting expensive) due to the drifters blowing them all up lol

cossie route your looking 3-5k dependent on what you want and will need hubs it may be easier to get the whole rear beam in there dependent on width, you could always buy a rotten 2wd keep the parts you want and break the rest