use an e30 i head on the eta block, that way you'll get dual valve springs and a better cam. This will lower the compression ratio to about 8:1, but since you're turboing it, you don't care much because you can crank the boost up.

I'd use an exhaust manifold off of a 324td or a 524td if you can find one. I don't know if I'd use the stock turbo off of the 524, but that's a good starting place. I'm not sure of the compatibility of the rest of the turbo gear from the 524, but that shouldn't be much of a problem...

I'd run with an e30 i ecu, but it'd need a custom chip programmed to deal with the unique fuel mapping of the turbo...